Method and apparatus for quantization

ABSTRACT

Disclosed herein are a video decoding method and apparatus and a video encoding method and apparatus. In quantization and dequantization, multiple quantization methods and multiple dequantization methods may be used. The multiple quantization methods include a variable-rate step quantization method and a fixed-rate step quantization method. The variable-rate step quantization method may be a quantization method in which an increment in a quantization step depending on an increase in a value of a quantization parameter by 1 is not fixed. The fixed-rate step quantization method may be a quantization method in which the increment in the quantization step depending on the increase of the value of the quantization parameter by 1 is fixed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application Nos.10-2017-0150664, filed Nov. 13, 2017 and 10-2018-0136853, filed Nov. 8,2018, which are hereby incorporated by reference in their entirety intothis application.

BACKGROUND OF THE INVENTION 1. Technical Field

The following embodiments relate generally to a video decoding methodand apparatus and a video encoding method and apparatus, and moreparticularly, to a method and apparatus that perform quantization invideo encoding and decoding.

2. Description of the Related Art

With the continuous development of the information and communicationindustries, broadcasting services supporting High-Definition (HD)resolution have been popularized all over the world. Through thispopularization, a large number of users have become accustomed tohigh-resolution and high-definition images and/or videos.

To satisfy users' demand for high definition, many institutions haveaccelerated the development of next-generation imaging devices. Users'interest in UHD TVs, having resolution that is more than four times ashigh as that of Full HD (FHD) TVs, as well as High-Definition TVs (HDTV)and FHD TVs, has increased. As interest therein has increased, imageencoding/decoding technology for images having higher resolution andhigher definition is continually required.

An image encoding/decoding apparatus and method may use inter-predictiontechnology, intra-prediction technology, entropy-coding technology, etc.so as to perform encoding/decoding on a high-resolution andhigh-definition image. Inter-prediction technology may be technology forpredicting the value of a pixel included in a target picture usingtemporally previous pictures and/or temporally subsequent pictures.Intra-prediction technology may be technology for predicting the valueof a pixel included in a target picture using information about pixelsin the target picture. Entropy-coding technology may be technology forassigning short code words to frequently occurring symbols and assigninglong code words to rarely occurring symbols.

In order to improve the efficiency of video encoding/decoding, varioustypes of quantization and inverse quantization (dequantization) methodshave been developed.

Video encoding/decoding technology can determine optimal codingparameters based on objective image quality. Determination as to the useof such objective image quality may improve the efficiency of videoencoding/decoding. However, determination as to the use of objectiveimage quality may cause results differing from those of perceptual imagequality (i.e. perceived image quality) actually experienced by viewers.In particular, when the bit rate of compressed data generated throughencoding is low, it may be difficult to control the amount ofinformation and perceptual image quality.

SUMMARY OF THE INVENTION

An embodiment is intended to provide an encoding apparatus and methodand a decoding apparatus and method which use scaling of a residualsignal.

An embodiment is intended to provide an encoding apparatus and methodand a decoding apparatus and method which determine a quantizationparameter and a quantization step.

In accordance with an aspect, there is provided an encoding method,including performing quantization that uses a quantization method,wherein the quantization method is a variable-rate step quantizationmethod, and wherein the variable-rate step quantization method is aquantization method in which an increment in a quantization stepdepending on an increase in a value of a quantization parameter by 1 isnot fixed.

As the value of the quantization parameter is smaller, the increment inthe quantization step of the variable-rate step quantization methoddepending on the increase in the value of the quantization parameter by1 may be larger.

The encoding method may further include selecting the quantizationmethod from among multiple quantization methods.

The multiple quantization methods may include a fixed-rate stepquantization method and the variable-rate step quantization method.

The fixed-rate step quantization method may be a quantization method inwhich the increment in the quantization step depending on the increasein the value of the quantization parameter by 1 is fixed.

The increment in the quantization step of the variable-rate stepquantization method may be less than the increment in the quantizationstep of the fixed-rate step quantization method in a region in which thevalue of the quantization parameter is relatively large.

In accordance with another aspect, there is provided a decoding method,including performing dequantization that uses a dequantization method,wherein the dequantization method is a variable-rate step dequantizationmethod, and wherein the variable-rate step dequantization method is adequantization method in which an increment in a quantization stepdepending on an increase in a value of a quantization parameter by 1 isnot fixed.

As the value of the quantization parameter is smaller, the increment inthe quantization step of the variable-rate step dequantization methoddepending on the increase in the value of the quantization parameter by1 may be larger.

The decoding method may further include selecting the dequantizationmethod from among multiple dequantization methods.

The multiple dequantization methods may include a fixed-rate stepdequantization method and the variable-rate step dequantization method.

The fixed-rate step dequantization method may be a dequantization methodin which the increment in the quantization step depending on theincrease in the value of the quantization parameter by 1 is fixed.

The increment in the quantization step of the variable-rate stepdequantization method may be less than the increment in the quantizationstep of the fixed-rate step dequantization method in a region in whichthe value of the quantization parameter is relatively large.

A quantization parameter of the fixed-rate step dequantization methodcorresponding to the quantization parameter of the variable-rate stepdequantization method may be determined.

A quantization step corresponding to the quantization parameter of thefixed-rate step dequantization method may be used for thedequantization.

A quantization step of the fixed-rate step dequantization methodcorresponding to the quantization parameter may be determined based onthe fixed-rate step dequantization method, and a quantization step ofthe variable-rate step dequantization method may be determined byapplying a quantization step rate to the quantization step of thefixed-rate step dequantization method.

The quantization step rate may be a ratio between the quantization stepof the variable-rate step dequantization method and the quantizationstep of the fixed-rate step dequantization method.

The decoding method may further include acquiring a quantization methodindicator.

The quantization method indicator may indicate one dequantization methodto be used for dequantization of a target block among the multipledequantization methods.

The quantization method indicator may indicate a dequantization methodto be applied to a specific unit.

The specific unit may be a video, a sequence, a picture or a slice.

The quantization method indicator may indicate whether the variable-ratestep dequantization method is applied to the specific unit.

The decoding method may further include acquiring the quantizationparameter using a quantization parameter difference value.

The quantization parameter may be used for the dequantization.

The quantization parameter difference value may be a difference betweena median value and the value of the quantization parameter.

The median value may be an intermediate value falling within a range ofthe quantization parameter of the variable-rate step dequantizationmethod.

The quantization parameter may be applied to a specific unit.

The decoding method may further include acquiring the quantizationparameter using a quantization parameter difference value.

The quantization parameter difference value may be a difference betweena value of a quantization parameter in an upper unit and a value of aquantization parameter in a lower unit.

The acquired quantization parameter may be used for dequantization ofthe lower unit.

The quantization parameter difference value may be included in a headerof the lower unit.

The upper unit may be a picture and the lower unit is a slice.

In accordance with a further aspect, there is provided acomputer-readable storage medium storing a bitstream for video decoding,the bitstream including information about a target block, whereindequantization that uses the information about the target block and andequantization method is performed, wherein the dequantization method isa variable-rate step dequantization method, and wherein thevariable-rate step dequantization method is a dequantization method inwhich an increment in a quantization step depending on an increase in avalue of a quantization parameter by 1 is not fixed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied;

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied;

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded;

FIG. 4 is a diagram illustrating the form of a Prediction Unit (PU) thata Coding Unit (CU) can include;

FIG. 5 is a diagram illustrating the form of a Transform Unit (TU) thatcan be included in a CU;

FIG. 6 illustrates splitting of a block according to an example;

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocedure;

FIG. 8 is a diagram for explaining the locations of reference samplesused in an intra-prediction procedure;

FIG. 9 is a diagram for explaining an embodiment of an inter-predictionprocedure;

FIG. 10 illustrates spatial candidates according to an embodiment;

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment;

FIG. 12 illustrates a transform and quantization process according to anexample;

FIG. 13 illustrates diagonal scanning according to an example;

FIG. 14 illustrates horizontal scanning according to an example;

FIG. 15 illustrates vertical scanning according to an example;

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment;

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment;

FIG. 18 illustrates traditional distortion and perceptual distortionaccording to an example;

FIG. 19 is a graph illustrating a relationship between the quantizationparameter and the quantization step of a fixed-rate step quantizationmethod according to an example;

FIG. 20 is a graph illustrating a relationship between the quantizationparameter and the SNR of the fixed-rate step quantization methodaccording to an example;

FIG. 21 is a flowchart of a quantization method according to anembodiment;

FIG. 22 is a flowchart of a dequantization method according to anembodiment;

FIG. 23 illustrates a quantization step depending on a quantizationparameter according to an example;

FIG. 24 illustrates a quantization step rate depending on a quantizationparameter according to an example;

FIG. 25 illustrates a table of quantization parameters and quantizationsteps in a variable-rate step quantization method according to anexample;

FIG. 26 illustrates the syntax of a video parameter set according to anexample;

FIG. 27 illustrates the syntax of a sequence parameter set according toan example; and

FIG. 28 illustrates the syntax of a picture parameter set according toan example.

FIG. 29 illustrates the syntax of a slice segment header according to anexample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be variously changed, and may have variousembodiments, and specific embodiments will be described in detail belowwith reference to the attached drawings. However, it should beunderstood that those embodiments are not intended to limit the presentinvention to specific disclosure forms, and that they include allchanges, equivalents or modifications included in the spirit and scopeof the present invention.

Detailed descriptions of the following exemplary embodiments will bemade with reference to the attached drawings illustrating specificembodiments. These embodiments are described so that those havingordinary knowledge in the technical field to which the presentdisclosure pertains can easily practice the embodiments. It should benoted that the various embodiments are different from each other, but donot need to be mutually exclusive of each other. For example, specificshapes, structures, and characteristics described here may beimplemented as other embodiments without departing from the spirit andscope of the embodiments in relation to an embodiment. Further, itshould be understood that the locations or arrangement of individualcomponents in each disclosed embodiment can be changed without departingfrom the spirit and scope of the embodiments. Therefore, theaccompanying detailed description is not intended to restrict the scopeof the disclosure, and the scope of the exemplary embodiments is limitedonly by the accompanying claims, along with equivalents thereof, as longas they are appropriately described.

In the drawings, similar reference numerals are used to designate thesame or similar functions in various aspects. The shapes, sizes, etc. ofcomponents in the drawings may be exaggerated to make the descriptionclear.

Terms such as “first” and “second” may be used to describe variouscomponents, but the components are not restricted by the terms. Theterms are used only to distinguish one component from another component.For example, a first component may be named a second component withoutdeparting from the scope of the present specification. Likewise, asecond component may be named a first component. The terms “and/or” mayinclude combinations of a plurality of related described items or any ofa plurality of related described items.

It will be understood that when a component is referred to as being“connected” or “coupled” to another component, the two components may bedirectly connected or coupled to each other, or intervening componentsmay be present between the two components. It will be understood thatwhen a component is referred to as being “directly connected orcoupled”, no intervening components are present between the twocomponents.

Also, components described in the embodiments are independently shown inorder to indicate different characteristic functions, but this does notmean that each of the components is formed of a separate piece ofhardware or software. That is, the components are arranged and includedseparately for convenience of description. For example, at least two ofthe components may be integrated into a single component. Conversely,one component may be divided into multiple components. An embodimentinto which the components are integrated or an embodiment in which somecomponents are separated is included in the scope of the presentspecification as long as it does not depart from the essence of thepresent specification.

Further, it should be noted that, in the exemplary embodiments, anexpression describing that a component “comprises” a specific componentmeans that additional components may be included within the scope of thepractice or the technical spirit of exemplary embodiments, but does notpreclude the presence of components other than the specific component.

The terms used in the present specification are merely used to describespecific embodiments and are not intended to limit the presentinvention. A singular expression includes a plural expression unless adescription to the contrary is specifically pointed out in context. Inthe present specification, it should be understood that the terms suchas “include” or “have” are merely intended to indicate that features,numbers, steps, operations, components, parts, or combinations thereofare present, and are not intended to exclude the possibility that one ormore other features, numbers, steps, operations, components, parts, orcombinations thereof will be present or added.

Embodiments will be described in detail below with reference to theaccompanying drawings so that those having ordinary knowledge in thetechnical field to which the embodiments pertain can easily practice theembodiments. In the following description of the embodiments, detaileddescriptions of known functions or configurations which are deemed tomake the gist of the present specification obscure will be omitted.Further, the same reference numerals are used to designate the samecomponents throughout the drawings, and repeated descriptions of thesame components will be omitted.

Hereinafter, “image” may mean a single picture constituting a video, ormay mean the video itself. For example, “encoding and/or decoding of animage” may mean “encoding and/or decoding of a video”, and may also mean“encoding and/or decoding of any one of images constituting the video”.

Hereinafter, the terms “video” and “motion picture” may be used to havethe same meaning, and may be used interchangeably with each other.

Hereinafter, a target image may be an encoding target image, which isthe target to be encoded, and/or a decoding target image, which is thetarget to be decoded. Further, the target image may be an input imagethat is input to an encoding apparatus or an input image that is inputto a decoding apparatus.

Hereinafter, the terms “image”, “picture”, “frame”, and “screen” may beused to have the same meaning and may be used interchangeably with eachother.

Hereinafter, a target block may be an encoding target block, i.e. thetarget to be encoded and/or a decoding target block, i.e. the target tobe decoded. Further, the target block may be a current block, i.e. thetarget to be currently encoded and/or decoded. Here, the terms “targetblock” and “current block” may be used to have the same meaning, and maybe used interchangeably with each other.

Hereinafter, the terms “block” and “unit” may be used to have the samemeaning, and may be used interchangeably with each other. Alternatively,“block” may denote a specific unit.

Hereinafter, the terms “region” and “segment” may be usedinterchangeably with each other.

Hereinafter, a specific signal may be a signal indicating a specificblock. For example, the original signal may be a signal indicating atarget block. A prediction signal may be a signal indicating aprediction block. A residual signal may be a signal indicating aresidual block.

In the following embodiments, specific information, data, a flag, anelement, and an attribute may have their respective values. A value of“0” corresponding to each of the information, data, flag, element, andattribute may indicate a logical false or a first predefined value. Inother words, the value of “0”, false, logical false, and a firstpredefined value may be used interchangeably with each other. A value of“1” corresponding to each of the information, data, flag, element, andattribute may indicate a logical true or a second predefined value. Inother words, the value of “1”, true, logical true, and a secondpredefined value may be used interchangeably with each other.

When a variable such as i or j is used to indicate a row, a column, oran index, the value of i may be an integer of 0 or more or an integer of1 or more. In other words, in the embodiments, each of a row, a column,and an index may be counted from 0 or may be counted from 1.

Below, the terms to be used in embodiments will be described.

Encoder: An encoder denotes a device for performing encoding.

Decoder: A decoder denotes a device for performing decoding.

Unit: A unit may denote the unit of image encoding and decoding. Theterms “unit” and “block” may be used to have the same meaning, and maybe used interchangeably with each other.

-   -   “Unit” may be an M×N array of samples. M and N may be positive        integers, respectively. The term “unit” may generally mean a        two-dimensional (2D) array of samples.    -   In the encoding and decoding of an image, “unit” may be an area        generated by the partitioning of one image. In other words,        “unit” may be a region specified in one image. A single image        may be partitioned into multiple units. Alternatively, one image        may be partitioned into sub-parts, and the unit may denote each        partitioned sub-part when encoding or decoding is performed on        the partitioned sub-part.    -   In the encoding and decoding of an image, predefined processing        may be performed on each unit depending on the type of the unit.    -   Depending on functions, the unit types may be classified into a        macro unit, a Coding Unit (CU), a Prediction Unit (PU), a        residual unit, a Transform Unit (TU), etc. Alternatively,        depending on functions, the unit may denote a block, a        macroblock, a coding tree unit, a coding tree block, a coding        unit, a coding block, a prediction unit, a prediction block, a        residual unit, a residual block, a transform unit, a transform        block, etc.    -   The term “unit” may mean information including a luminance        (luma) component block, a chrominance (chroma) component block        corresponding thereto, and syntax elements for respective blocks        so that the unit is designated to be distinguished from a block.    -   The size and shape of a unit may be variously implemented.        Further, a unit may have any of various sizes and shapes. In        particular, the shapes of the unit may include not only a        square, but also a geometric figure that can be represented in        two dimensions (2D), such as a rectangle, a trapezoid, a        triangle, and a pentagon.    -   Further, unit information may include one or more of the type of        a unit, the size of a unit, the depth of a unit, the order of        encoding of a unit and the order of decoding of a unit, etc. For        example, the type of a unit may indicate one of a CU, a PU, a        residual unit and a TU.    -   One unit may be partitioned into sub-units, each having a        smaller size than that of the relevant unit.    -   Depth: A depth may denote the degree to which the unit is        partitioned. Further, the unit depth may indicate the level at        which the corresponding unit is present when units are        represented in a tree structure.    -   Unit partition information may include a depth indicating the        depth of a unit. A depth may indicate the number of times the        unit is partitioned and/or the degree to which the unit is        partitioned.    -   In a tree structure, it may be considered that the depth of a        root node is the smallest, and the depth of a leaf node is the        largest.    -   A single unit may be hierarchically partitioned into multiple        sub-units while having depth information based on a tree        structure. In other words, the unit and sub-units, generated by        partitioning the unit, may correspond to a node and child nodes        of the node, respectively. Each of the partitioned sub-units may        have a unit depth. Since the depth indicates the number of times        the unit is partitioned and/or the degree to which the unit is        partitioned, the partition information of the sub-units may        include information about the sizes of the sub-units.    -   In a tree structure, the top node may correspond to the initial        node before partitioning. The top node may be referred to as a        “root node”. Further, the root node may have a minimum depth        value. Here, the top node may have a depth of level ‘0’    -   A node having a depth of level ‘1’ may denote a unit generated        when the initial unit is partitioned once. A node having a depth        of level ‘2’ may denote a unit generated when the initial unit        is partitioned twice.    -   A leaf node having a depth of level ‘n’ may denote a unit        generated when the initial unit has been partitioned n times.    -   The leaf node may be a bottom node, which cannot be partitioned        any further. The depth of the leaf node may be the maximum        level. For example, a predefined value for the maximum level may        be 3.    -   A QT depth may denote a depth for a quad-partitioning. A BT        depth may denote a depth for a binary-partitioning. A TT depth        may denote a depth for a ternary-partitioning.

Sample: A sample may be a base unit constituting a block. A sample maybe represented by values from 0 to 2^(Bd-)1 depending on the bit depth(Bd).

-   -   A sample may be a pixel or a pixel value.    -   Hereinafter, the terms “pixel” and “sample” may be used to have        the same meaning, and may be used interchangeably with each        other.

A Coding Tree Unit (CTU): A CTU may be composed of a single lumacomponent (Y) coding tree block and two chroma component (Cb, Cr) codingtree blocks related to the luma component coding tree block. Further, aCTU may mean information including the above blocks and a syntax elementfor each of the blocks.

-   -   Each coding tree unit (CTU) may be partitioned using one or more        partitioning methods, such as a quad tree (QT), a binary tree        (BT), and a ternary tree (TT) so as to configure sub-units, such        as a coding unit, a prediction unit, and a transform unit.    -   “CTU” may be used as a term designating a pixel block, which is        a processing unit in an image-decoding and encoding process, as        in the case of partitioning of an input image.

Coding Tree Block (CTB): “CTB” may be used as a term designating any oneof a Y coding tree block, a Cb coding tree block, and a Cr coding treeblock.

Neighbor block: A neighbor block (or neighboring block) may mean a blockadjacent to a target block. A neighbor block may mean a reconstructedneighbor block.

Hereinafter, the terms “neighbor block” and “adjacent block” may be usedto have the same meaning and may be used interchangeably with eachother.

Spatial neighbor block; A spatial neighbor block may a block spatiallyadjacent to a target block. A neighbor block may include a spatialneighbor block.

-   -   The target block and the spatial neighbor block may be included        in a target picture.    -   The spatial neighbor block may mean a block, the boundary of        which is in contact with the target block, or a block located        within a predetermined distance from the target block.    -   The spatial neighbor block may mean a block adjacent to the        vertex of the target block. Here, the block adjacent to the        vertex of the target block may mean a block vertically adjacent        to a neighbor block which is horizontally adjacent to the target        block or a block horizontally adjacent to a neighbor block which        is vertically adjacent to the target block.

Temporal neighbor block: A temporal neighbor block may be a blocktemporally adjacent to a target block. A neighbor block may include atemporal neighbor block.

-   -   The temporal neighbor block may include a co-located block (col        block).    -   The col block may be a block in a previously reconstructed        co-located picture (col picture). The location of the col block        in the col-picture may correspond to the location of the target        block in a target picture. The col picture may be a picture        included in a reference picture list.    -   The temporal neighbor block may be a block temporally adjacent        to a spatial neighbor block of a target block.

Prediction unit: A prediction unit may be a base unit for prediction,such as inter prediction, intra prediction, inter compensation, intracompensation, and motion compensation.

-   -   A single prediction unit may be divided into multiple partitions        having smaller sizes or sub-prediction units. The multiple        partitions may also be base units in the performance of        prediction or compensation. The partitions generated by dividing        the prediction unit may also be prediction units.

Prediction unit partition: A prediction unit partition may be the shapeinto which a prediction unit is divided.

Reconstructed neighboring unit: A reconstructed neighboring unit may bea unit which has already been decoded and reconstructed around a targetunit.

-   -   A reconstructed neighboring unit may be a unit that is spatially        adjacent to the target unit or that is temporally adjacent to        the target unit.    -   A reconstructed spatially neighboring unit may be a unit which        is included in a target picture and which has already been        reconstructed through encoding and/or decoding.    -   A reconstructed temporally neighboring unit may be a unit which        is included in a reference image and which has already been        reconstructed through encoding and/or decoding. The location of        the reconstructed temporally neighboring unit in the reference        image may be identical to that of the target unit in the target        picture, or may correspond to the location of the target unit in        the target picture.

Parameter set: A parameter set may be header information in thestructure of a bitstream. For example, a parameter set may include avideo parameter set, a sequence parameter set, a picture parameter set,an adaptation parameter set, etc.

Further, the parameter set may include slice header information and tileheader information.

Rate-distortion optimization: An encoding apparatus may userate-distortion optimization so as to provide high coding efficiency byutilizing combinations of the size of a coding unit (CU), a predictionmode, the size of a prediction unit (PU), motion information, and thesize of a transform unit (TU).

-   -   A rate-distortion optimization scheme may calculate        rate-distortion costs of respective combinations so as to select        an optimal combination from among the combinations. The        rate-distortion costs may be calculated using the following        Equation 1. Generally, a combination enabling the        rate-distortion cost to be minimized may be selected as the        optimal combination in the rate-distortion optimization scheme.

D+λ*R  [Equation 1]

-   -   D may denote distortion. D may be the mean of squares of        differences (i.e. mean square error) between original transform        coefficients and reconstructed transform coefficients in a        transform unit.    -   R may denote the rate, which may denote a bit rate using        related-context information.    -   λ denotes a Lagrangian multiplier. R may include not only coding        parameter information, such as a prediction mode, motion        information, and a coded block flag, but also bits generated due        to the encoding of transform coefficients.    -   An encoding apparatus may perform procedures, such as inter        prediction and/or intra prediction, transform, quantization,        entropy encoding, inverse quantization (dequantization), and        inverse transform so as to calculate precise D and R. These        procedures may greatly increase the complexity of the encoding        apparatus.    -   Bitstream: A bitstream may denote a stream of bits including        encoded image information.    -   Parameter set: A parameter set may be header information in the        structure of a bitstream.    -   The parameter set may include at least one of a video parameter        set, a sequence parameter set, a picture parameter set, and an        adaptation parameter set. Further, the parameter set may include        information about a slice header and information about a tile        header.

Parsing: Parsing may be the decision on the value of a syntax element,made by performing entropy decoding on a bitstream. Alternatively, theterm “parsing” may mean such entropy decoding itself.

Symbol: A symbol may be at least one of the syntax element, the codingparameter, and the transform coefficient of an encoding target unitand/or a decoding target unit. Further, a symbol may be the target ofentropy encoding or the result of entropy decoding.

Reference picture: A reference picture may be an image referred to by aunit so as to perform inter prediction or motion compensation.Alternatively, a reference picture may be an image including a referenceunit referred to by a target unit so as to perform inter prediction ormotion compensation.

Hereinafter, the terms “reference picture” and “reference image” may beused to have the same meaning, and may be used interchangeably with eachother.

Reference picture list: A reference picture list may be a list includingone or more reference images used for inter prediction or motioncompensation.

-   -   The types of a reference picture list may include List Combined        (LC), List 0 (L0), List 1 (L1), List 2 (L2), List 3 (L3), etc.    -   For inter prediction, one or more reference picture lists may be        used.

Inter-prediction indicator: An inter-prediction indicator may indicatethe inter-prediction direction for a target unit. Inter prediction maybe one of unidirectional prediction and bidirectional prediction.Alternatively, the inter-prediction indicator may denote the number ofreference images used to generate a prediction unit of a target unit.Alternatively, the inter-prediction indicator may denote the number ofprediction blocks used for inter prediction or motion compensation of atarget unit.

Reference picture index: A reference picture index may be an indexindicating a specific reference image in a reference picture list.

Motion vector (MV): A motion vector may be a 2D vector used for interprediction or motion compensation. A motion vector may mean an offsetbetween a target image and a reference image.

-   -   For example, a MV may be represented in a form such as (mv_(x),        mv_(y)). mv_(x) may indicate a horizontal component, and mv_(y)        may indicate a vertical component.    -   Search range: A search range may be a 2D area in which a search        for a MV is performed during inter prediction. For example, the        size of the search range may be M×N. M and N may be respective        positive integers.

Motion vector candidate: A motion vector candidate may be a block thatis a prediction candidate or the motion vector of the block that is aprediction candidate when a motion vector is predicted.

-   -   A motion vector candidate may be included in a motion vector        candidate list.

Motion vector candidate list: A motion vector candidate list may be alist configured using one or more motion vector candidates.

Motion vector candidate index: A motion vector candidate index may be anindicator for indicating a motion vector candidate in the motion vectorcandidate list. Alternatively, a motion vector candidate index may bethe index of a motion vector predictor.

Motion information: Motion information may be information including atleast one of a reference picture list, a reference image, a motionvector candidate, a motion vector candidate index, a merge candidate,and a merge index, as well as a motion vector, a reference pictureindex, and an inter-prediction indicator.

Merge candidate list: A merge candidate list may be a list configuredusing merge candidates.

Merge candidate: A merge candidate may be a spatial merge candidate, atemporal merge candidate, a combined merge candidate, a combinedbi-prediction merge candidate, a zero-merge candidate, etc. A mergecandidate may include motion information such as prediction typeinformation, a reference picture index for each list, and a motionvector.

Merge index: A merge index may be an indicator for indicating a mergecandidate in a merge candidate list.

-   -   A merge index may indicate a reconstructed unit used to derive a        merge candidate between a reconstructed unit spatially adjacent        to a target unit and a reconstructed unit temporally adjacent to        the target unit.    -   A merge index may indicate at least one of pieces of motion        information of a merge candidate.

Transform unit: A transform unit may be the base unit of residual signalencoding and/or residual signal decoding, such as transform, inversetransform, quantization, dequantization, transform coefficient encoding,and transform coefficient decoding. A single transform unit may bepartitioned into multiple transform units having smaller sizes.

Scaling: Scaling may denote a procedure for multiplying a factor by atransform coefficient level.

-   -   As a result of scaling of the transform coefficient level, a        transform coefficient may be generated. Scaling may also be        referred to as “dequantization”.

Quantization Parameter (QP): A quantization parameter may be a valueused to generate a transform coefficient level for a transformcoefficient in quantization. Alternatively, a quantization parameter mayalso be a value used to generate a transform coefficient by scaling thetransform coefficient level in dequantization. Alternatively, aquantization parameter may be a value mapped to a quantization stepsize.

Delta quantization parameter: A delta quantization parameter is adifferential value between a predicted quantization parameter and thequantization parameter of a target unit.

Scan: Scan may denote a method for aligning the order of coefficients ina unit, a block or a matrix. For example, a method for aligning a 2Darray in the form of a one-dimensional (1D) array may be referred to asa “scan”. Alternatively, a method for aligning a 1D array in the form ofa 2D array may also be referred to as a “scan” or an “inverse scan”.

Transform coefficient: A transform coefficient may be a coefficientvalue generated as an encoding apparatus performs a transform.Alternatively, the transform coefficient may be a coefficient valuegenerated as a decoding apparatus performs at least one of entropydecoding and dequantization.

-   -   A quantized level or a quantized transform coefficient level        generated by applying quantization to a transform coefficient or        a residual signal may also be included in the meaning of the        term “transform coefficient”.

Quantized level: A quantized level may be a value generated as theencoding apparatus performs quantization on a transform coefficient or aresidual signal. Alternatively, the quantized level may be a value thatis the target of dequantization as the decoding apparatus performsdequantization.

-   -   A quantized transform coefficient level, which is the result of        transform and quantization, may also be included in the meaning        of a quantized level.

Non-zero transform coefficient: A non-zero transform coefficient may bea transform coefficient having a value other than 0 or a transformcoefficient level having a value other than 0. Alternatively, a non-zerotransform coefficient may be a transform coefficient, the magnitude ofthe value of which is not 0, or a transform coefficient level, themagnitude of the value of which is not 0.

Quantization matrix: A quantization matrix may be a matrix used in aquantization procedure or a dequantization procedure so as to improvethe subjective image quality or objective image quality of an image. Aquantization matrix may also be referred to as a “scaling list”.

Quantization matrix coefficient: A quantization matrix coefficient maybe each element in a quantization matrix. A quantization matrixcoefficient may also be referred to as a “matrix coefficient”.

Default matrix: A default matrix may be a quantization matrix predefinedby the encoding apparatus and the decoding apparatus.

Non-default matrix: A non-default matrix may be a quantization matrixthat is not predefined by the encoding apparatus and the decodingapparatus. The non-default matrix may be signaled by the encodingapparatus to the decoding apparatus.

Signaling: “signaling” may denote that information is transferred froman encoding apparatus to a decoding apparatus. Alternatively,“signaling” may mean information is included in in a bitstream or arecoding medium. Information signaled by an encoding apparatus may beused by a decoding apparatus.

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatusor an image encoding apparatus. A video may include one or more images(pictures). The encoding apparatus 100 may sequentially encode one ormore images of the video.

Referring to FIG. 1, the encoding apparatus 100 includes aninter-prediction unit 110, an intra-prediction unit 120, a switch 115, asubtractor 125, a transform unit 130, a quantization unit 140, anentropy encoding unit 150, a dequantization (inverse quantization) unit160, an inverse transform unit 170, an adder 175, a filter unit 180, anda reference picture buffer 190.

The encoding apparatus 100 may perform encoding on a target image usingan intra mode and/or an inter mode.

Further, the encoding apparatus 100 may generate a bitstream, includinginformation about encoding, via encoding on the target image, and mayoutput the generated bitstream. The generated bitstream may be stored ina computer-readable storage medium and may be streamed through awired/wireless transmission medium.

When the intra mode is used as a prediction mode, the switch 115 mayswitch to the intra mode. When the inter mode is used as a predictionmode, the switch 115 may switch to the inter mode.

The encoding apparatus 100 may generate a prediction block of a targetblock. Further, after the prediction block has been generated, theencoding apparatus 100 may encode a residual between the target blockand the prediction block.

When the prediction mode is the intra mode, the intra-prediction unit120 may use pixels of previously encoded/decoded neighboring blocksaround the target block as reference samples. The intra-prediction unit120 may perform spatial prediction on the target block using thereference samples, and may generate prediction samples for the targetblock via spatial prediction.

The inter-prediction unit 110 may include a motion prediction unit and amotion compensation unit.

When the prediction mode is an inter mode, the motion prediction unitmay search a reference image for the area most closely matching thetarget block in a motion prediction procedure, and may derive a motionvector for the target block and the found area based on the found area.

The reference image may be stored in the reference picture buffer 190.More specifically, the reference image may be stored in the referencepicture buffer 190 when the encoding and/or decoding of the referenceimage have been processed.

The motion compensation unit may generate a prediction block for thetarget block by performing motion compensation using a motion vector.Here, the motion vector may be a two-dimensional (2D) vector used forinter-prediction. Further, the motion vector may indicate an offsetbetween the target image and the reference image.

The motion prediction unit and the motion compensation unit may generatea prediction block by applying an interpolation filter to a partial areaof a reference image when the motion vector has a value other than aninteger. In order to perform inter prediction or motion compensation, itmay be determined which one of a skip mode, a merge mode, an advancedmotion vector prediction (AMVP) mode, and a current picture referencemode corresponds to a method for predicting the motion of a PU includedin a CU, based on the CU, and compensating for the motion, and interprediction or motion compensation may be performed depending on themode.

The subtractor 125 may generate a residual block, which is thedifferential between the target block and the prediction block. Aresidual block may also be referred to as a “residual signal”.

The residual signal may be the difference between an original signal anda prediction signal. Alternatively, the residual signal may be a signalgenerated by transforming or quantizing the difference between anoriginal signal and a prediction signal or by transforming andquantizing the difference. A residual block may be a residual signal fora block unit.

The transform unit 130 may generate a transform coefficient bytransforming the residual block, and may output the generated transformcoefficient. Here, the transform coefficient may be a coefficient valuegenerated by transforming the residual block.

The transform unit 130 may use one of multiple predefined transformmethods when performing a transform.

The multiple predefined transform methods may include a Discrete CosineTransform (DCT), a Discrete Sine Transform (DST), a Karhunen-LoeveTransform (KLT), etc.

The transform method used to transform a residual block may bedetermined depending on at least one of coding parameters for a targetblock and/or a neighboring block. For example, the transform method maybe determined based on at least one of an inter-prediction mode for aPU, an intra-prediction mode for a PU, the size of a TU, and the shapeof a TU. Alternatively, transformation information indicating thetransform method may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

When a transform skip mode is used, the transform unit 130 may omittransforming the residual block.

By applying quantization to the transform coefficient, a quantizedtransform coefficient level or a quantized level may be generated.Hereinafter, in the embodiments, each of the quantized transformcoefficient level and the quantized level may also be referred to as a‘transform coefficient’.

The quantization unit 140 may generate a quantized transform coefficientlevel or a quantized level by quantizing the transform coefficientdepending on quantization parameters. The quantization unit 140 mayoutput the quantized transform coefficient level or the quantized levelthat is generated. In this case, the quantization unit 140 may quantizethe transform coefficient using a quantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingprobability distribution-based entropy encoding based on values,calculated by the quantization unit 140, and/or coding parameter values,calculated in the encoding procedure. The entropy encoding unit 150 mayoutput the generated bitstream.

The entropy encoding unit 150 may perform entropy encoding oninformation about the pixels of the image and information required todecode the image. For example, the information required to decode theimage may include syntax elements or the like.

When entropy encoding is applied, fewer bits may be assigned to morefrequently occurring symbols, and more bits may be assigned to rarelyoccurring symbols. As symbols are represented by means of thisassignment, the size of a bit string for target symbols to be encodedmay be reduced. Therefore, the compression performance of video encodingmay be improved through entropy encoding.

Further, for entropy encoding, the entropy encoding unit 150 may use acoding method such as exponential Golomb, Context-Adaptive VariableLength Coding (CAVLC), or Context-Adaptive Binary Arithmetic Coding(CABAC). For example, the entropy encoding unit 150 may perform entropyencoding using a Variable Length Coding/Code (VLC) table. For example,the entropy encoding unit 150 may derive a binarization method for atarget symbol. Further, the entropy encoding unit 150 may derive aprobability model for a target symbol/bin. The entropy encoding unit 150may perform arithmetic coding using the derived binarization method, aprobability model, and a context model.

The entropy encoding unit 150 may transform the coefficient of the formof a 2D block into the form of a 1D vector through a transformcoefficient scanning method so as to encode a quantized transformcoefficient level.

The coding parameters may be information required for encoding and/ordecoding. The coding parameters may include information encoded by theencoding apparatus 100 and transferred from the encoding apparatus 100to a decoding apparatus, and may also include information that may bederived in the encoding or decoding procedure. For example, informationtransferred to the decoding apparatus may include syntax elements.

The coding parameters may include not only information (or a flag or anindex), such as a syntax element, which is encoded by the encodingapparatus and is signaled by the encoding apparatus to the decodingapparatus, but also information derived in an encoding or decodingprocess. Further, the coding parameters may include information requiredso as to encode or decode images. For example, the coding parameters mayinclude at least one value, combinations or statistics of the size of aunit/block, the depth of a unit/block, partition information of aunit/block, the partition structure of a unit/block, informationindicating whether a unit/block is partitioned in a quad-tree structure,information indicating whether a unit/block is partitioned in a binarytree structure, the partitioning direction of a binary tree structure(horizontal direction or vertical direction), the partitioning form of abinary tree structure (symmetrical partitioning or asymmetricalpartitioning), information indicating whether a unit/block ispartitioned in a ternary tree structure, the partitioning direction of aternary tree structure (horizontal direction or vertical direction), aprediction scheme (intra prediction or inter prediction), anintra-prediction mode/direction, a reference sample filtering method, aprediction block filtering method, a prediction block boundary filteringmethod, a filter tap for filtering, a filter coefficient for filtering,an inter-prediction mode, motion information, a motion vector, areference picture index, an inter-prediction direction, aninter-prediction indicator, a reference picture list, a reference image,a motion vector predictor, a motion vector prediction candidate, amotion vector candidate list, information indicating whether a mergemode is used, a merge candidate, a merge candidate list, informationindicating whether a skip mode is used, the type of an interpolationfilter, the tap of an interpolation filter, the filter coefficient of aninterpolation filter, the magnitude of a motion vector, accuracy ofmotion vector representation, a transform type, a transform size,information indicating whether a primary transform is used, informationindicating whether an additional (secondary) transform is used, aprimary transform index, a secondary transform index, informationindicating the presence or absence of a residual signal, a coded blockpattern, a coded block flag, a quantization parameter, a quantizationmatrix, information about an intra-loop filter, information indicatingwhether an intra-loop filter is applied, the coefficient of anintra-loop filter, the tap of an intra-loop filter, the shape/form of anintra-loop filter, information indicating whether a deblocking filter isapplied, the coefficient of a deblocking filter, the tap of a deblockingfilter, deblocking filter strength, the shape/form of a deblockingfilter, information indicating whether an adaptive sample offset isapplied, the value of an adaptive sample offset, the category of anadaptive sample offset, the type of an adaptive sample offset,information indicating whether an adaptive in-loop filter is applied,the coefficient of an adaptive in-loop filter, the tap of an adaptivein-loop filter, the shape/form of an adaptive in-loop filter, abinarization/inverse binarization method, a context model, a contextmodel decision method, a context model update method, informationindicating whether a regular mode is performed, information whether abypass mode is performed, a context bin, a bypass bin, a transformcoefficient, a transform coefficient level, a transform coefficientlevel scanning method, an image display/output order, sliceidentification information, a slice type, slice partition information,tile identification information, a tile type, tile partitioninformation, a picture type, bit depth, information about a luma signal,and information about a chroma signal. The prediction scheme may denoteone prediction mode of an intra prediction mode and an inter predictionmode.

The residual signal may denote the difference between the originalsignal and a prediction signal. Alternatively, the residual signal maybe a signal generated by transforming the difference between theoriginal signal and the prediction signal. Alternatively, the residualsignal may be a signal generated by transforming and quantizing thedifference between the original signal and the prediction signal. Aresidual block may be the residual signal for a block.

Here, signaling a flag or an index may mean that the encoding apparatus100 includes an entropy-encoded flag or an entropy-encoded index,generated by performing entropy encoding on the flag or index, in abitstream, and that the decoding apparatus 200 acquires a flag or anindex by performing entropy decoding on the entropy-encoded flag or theentropy-encoded index, extracted from the bitstream.

Since the encoding apparatus 100 performs encoding via inter prediction,the encoded target image may be used as a reference image for additionalimage(s) to be subsequently processed. Therefore, the encoding apparatus100 may reconstruct or decode the encoded target image and store thereconstructed or decoded image as a reference image in the referencepicture buffer 190. For decoding, dequantization and inverse transformon the encoded target image may be processed.

The quantized level may be inversely quantized by the dequantizationunit 160, and may be inversely transformed by the inverse transform unit170. The coefficient that has been inversely quantized and/or inverselytransformed may be added to the prediction block by the adder 175. Theinversely quantized and/or inversely transformed coefficient and theprediction block are added, and then a reconstructed block may begenerated. Here, the inversely quantized and/or inversely transformedcoefficient may denote a coefficient on which one or more ofdequantization and inverse transform are performed, and may also denotea reconstructed residual block.

The reconstructed block may be subjected to filtering through the filterunit 180. The filter unit 180 may apply one or more of a deblockingfilter, a Sample Adaptive Offset (SAO) filter, and an Adaptive LoopFilter (ALF) to the reconstructed block or a reconstructed picture. Thefilter unit 180 may also be referred to as an “in-loop filter”.

The deblocking filter may eliminate block distortion occurring at theboundaries between blocks. In order to determine whether to apply thedeblocking filter, the number of columns or rows which are included in ablock and which include pixel(s) based on which it is determined whetherto apply the deblocking filter to a target block may be decided on.

When the deblocking filter is applied to the target block, the appliedfilter may differ depending on the strength of the required deblockingfiltering. In other words, among different filters, a filter decided onin consideration of the strength of deblocking filtering may be appliedto the target block. When a deblocking filter is applied to a targetblock, a filter corresponding to any one of a strong filter and a weakfilter may be applied to the target block depending on the strength ofrequired deblocking filtering.

Also, when vertical filtering and horizontal filtering are performed onthe target block, the horizontal filtering and the vertical filteringmay be processed in parallel.

The SAO may add a suitable offset to the values of pixels to compensatefor coding error. The SAO may perform, for the image to which deblockingis applied, correction that uses an offset in the difference between anoriginal image and the image to which deblocking is applied, on a pixelbasis. To perform an offset correction for an image, a method fordividing the pixels included in the image into a certain number ofregions, determining a region to which an offset is to be applied, amongthe divided regions, and applying an offset to the determined region maybe used, and a method for applying an offset in consideration of edgeinformation of each pixel may also be used.

The ALF may perform filtering based on a value obtained by comparing areconstructed image with an original image. After pixels included in animage have been divided into a predetermined number of groups, filtersto be applied to each group may be determined, and filtering may bedifferentially performed for respective groups. For a luma signal,information related to whether to apply an adaptive loop filter may besignaled for each CU. The shapes and filter coefficients of ALFs to beapplied to respective blocks may differ for respective blocks.Alternatively, regardless of the features of a block, an ALF having afixed form may be applied to the block.

The reconstructed block or the reconstructed image subjected tofiltering through the filter unit 180 may be stored in the referencepicture buffer 190. The reconstructed block subjected to filteringthrough the filter unit 180 may be a part of a reference picture. Inother words, the reference picture may be a reconstructed picturecomposed of reconstructed blocks subjected to filtering through thefilter unit 180. The stored reference picture may be subsequently usedfor inter prediction.

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied.

A decoding apparatus 200 may be a decoder, a video decoding apparatus oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization (inverse quantization) unit 220, aninverse transform unit 230, an intra-prediction unit 240, aninter-prediction unit 250, a switch 245 an adder 255, a filter unit 260,and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer-readable storage medium, and may receivea bitstream that is streamed through a wired/wireless transmissionmedium.

The decoding apparatus 200 may perform decoding on the bitstream in anintra mode and/or an inter mode. Further, the decoding apparatus 200 maygenerate a reconstructed image or a decoded image via decoding, and mayoutput the reconstructed image or decoded image.

For example, switching to an intra mode or an inter mode based on theprediction mode used for decoding may be performed by the switch 245.When the prediction mode used for decoding is an intra mode, the switch245 may be operated to switch to the intra mode. When the predictionmode used for decoding is an inter mode, the switch 245 may be operatedto switch to the inter mode.

The decoding apparatus 200 may acquire a reconstructed residual block bydecoding the input bitstream, and may generate a prediction block. Whenthe reconstructed residual block and the prediction block are acquired,the decoding apparatus 200 may generate a reconstructed block, which isthe target to be decoded, by adding the reconstructed residual block tothe prediction block.

The entropy decoding unit 210 may generate symbols by performing entropydecoding on the bitstream based on the probability distribution of abitstream. The generated symbols may include quantized transformcoefficient level-format symbols. Here, the entropy decoding method maybe similar to the above-described entropy encoding method. That is, theentropy decoding method may be the reverse procedure of theabove-described entropy encoding method.

The entropy decoding unit 210 may change a coefficient having aone-dimensional (1D) vector form to a 2D block shape through a transformcoefficient scanning method in order to decode a quantized transformcoefficient level.

For example, the coefficients of the block may be changed to 2D blockshapes by scanning the block coefficients using up-right diagonalscanning. Alternatively, which one of up-right diagonal scanning,vertical scanning, and horizontal scanning is to be used may bedetermined depending on the size and/or the intra-prediction mode of thecorresponding block.

The quantized coefficient may be inversely quantized by thedequantization unit 220. The dequantization unit 220 may generate aninversely quantized coefficient by performing dequantization on thequantized coefficient. Further, the inversely quantized coefficient maybe inversely transformed by the inverse transform unit 230. The inversetransform unit 230 may generate a reconstructed residual block byperforming an inverse transform on the inversely quantized coefficient.As a result of performing dequantization and the inverse transform onthe quantized coefficient, the reconstructed residual block may begenerated. Here, the dequantization unit 220 may apply a quantizationmatrix to the quantized coefficient when generating the reconstructedresidual block.

When the intra mode is used, the intra-prediction unit 240 may generatea prediction block by performing spatial prediction that uses the pixelvalues of previously decoded neighboring blocks around a target block.

The inter-prediction unit 250 may include a motion compensation unit.Alternatively, the inter-prediction unit 250 may be designated as a“motion compensation unit”.

When the inter mode is used, the motion compensation unit may generate aprediction block by performing motion compensation that uses a motionvector and a reference image stored in the reference picture buffer 270.

The motion compensation unit may apply an interpolation filter to apartial area of the reference image when the motion vector has a valueother than an integer, and may generate a prediction block using thereference image to which the interpolation filter is applied. In orderto perform motion compensation, the motion compensation unit maydetermine which one of a skip mode, a merge mode, an Advanced MotionVector Prediction (AMVP) mode, and a current picture reference modecorresponds to the motion compensation method used for a PU included ina CU, based on the CU, and may perform motion compensation depending onthe determined mode.

The reconstructed residual block and the prediction block may be addedto each other by the adder 255. The adder 255 may generate areconstructed block by adding the reconstructed residual block to theprediction block.

The reconstructed block may be subjected to filtering through the filterunit 260. The filter unit 260 may apply at least one of a deblockingfilter, an SAO filter, and an ALF to the reconstructed block or thereconstructed image. The reconstructed image may be a picture includingthe reconstructed block.

The reconstructed image subjected to filtering may be outputted by theencoding apparatus 100, and may be used by the encoding apparatus.

The reconstructed image subjected to filtering through the filter unit260 may be stored as a reference picture in the reference picture buffer270. The reconstructed block subjected to filtering through the filterunit 260 may be a part of the reference picture. In other words, thereference picture may be an image composed of reconstructed blockssubjected to filtering through the filter unit 260. The stored referencepicture may be subsequently used for inter prediction.

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded.

FIG. 3 may schematically illustrate an example in which a single unit ispartitioned into multiple sub-units.

In order to efficiently partition the image, a Coding Unit (CU) may beused in encoding and decoding. The term “unit” may be used tocollectively designate 1) a block including image samples and 2) asyntax element. For example, the “partitioning of a unit” may mean the“partitioning of a block corresponding to a unit”.

A CU may be used as a base unit for image encoding/decoding. A CU may beused as a unit to which one mode selected from an intra mode and aninter mode in image encoding/decoding is applied. In other words, inimage encoding/decoding, which one of an intra mode and an inter mode isto be applied to each CU may be determined.

Further, a CU may be a base unit in prediction, transform, quantization,inverse transform, dequantization, and encoding/decoding of transformcoefficients.

Referring to FIG. 3, an image 200 may be sequentially partitioned intounits corresponding to a Largest Coding Unit (LCU), and a partitionstructure may be determined for each LCU. Here, the LCU may be used tohave the same meaning as a Coding Tree Unit (CTU).

The partitioning of a unit may mean the partitioning of a blockcorresponding to the unit. Block partition information may include depthinformation about the depth of a unit. The depth information mayindicate the number of times the unit is partitioned and/or the degreeto which the unit is partitioned. A single unit may be hierarchicallypartitioned into sub-units while having depth information based on atree structure. Each of partitioned sub-units may have depthinformation. The depth information may be information indicating thesize of a CU. The depth information may be stored for each CU.

Each CU may have depth information. When the CU is partitioned, CUsresulting from partitioning may have a depth increased from the depth ofthe partitioned CU by 1.

The partition structure may mean the distribution of Coding Units (CUs)to efficiently encode the image in an LCU 310. Such a distribution maybe determined depending on whether a single CU is to be partitioned intomultiple CUs. The number of CUs generated by partitioning may be apositive integer of 2 or more, including 2, 3, 4, 8, 16, etc. Thehorizontal size and the vertical size of each of CUs generated by thepartitioning may be less than the horizontal size and the vertical sizeof a CU before being partitioned, depending on the number of CUsgenerated by partitioning.

Each partitioned CU may be recursively partitioned into four CUs in thesame way. Via the recursive partitioning, at least one of the horizontalsize and the vertical size of each partitioned CU may be reducedcompared to at least one of the horizontal size and the vertical size ofthe CU before being partitioned.

The partitioning of a CU may be recursively performed up to a predefineddepth or a predefined size. For example, the depth of a CU may have avalue ranging from 0 to 3. The size of the CU may range from a size of64×64 to a size of 8×8 depending on the depth of the CU.

For example, the depth of an LCU may be 0, and the depth of a SmallestCoding Unit (SCU) may be a predefined maximum depth. Here, as describedabove, the LCU may be the CU having the maximum coding unit size, andthe SCU may be the CU having the minimum coding unit size.

Partitioning may start at the LCU 310, and the depth of a CU may beincreased by 1 whenever the horizontal and/or vertical sizes of the CUare reduced by partitioning.

For example, for respective depths, a CU that is not partitioned mayhave a size of 2N×2N. Further, in the case of a CU that is partitioned,a CU having a size of 2N×2N may be partitioned into four CUs, eachhaving a size of N×N. The value of N may be halved whenever the depth isincreased by 1.

Referring to FIG. 3, an LCU having a depth of 0 may have 64×64 pixels or64×64 blocks. 0 may be a minimum depth. An SCU having a depth of 3 mayhave 8×8 pixels or 8×8 blocks. 3 may be a maximum depth. Here, a CUhaving 64×64 blocks, which is the LCU, may be represented by a depth of0. A CU having 32×32 blocks may be represented by a depth of 1. A CUhaving 16×16 blocks may be represented by a depth of 2. A CU having 8×8blocks, which is the SCU, may be represented by a depth of 3.

Information about whether the corresponding CU is partitioned may berepresented by the partition information of the CU. The partitioninformation may be 1-bit information. All CUs except the SCU may includepartition information. For example, the value of the partitioninformation of a CU that is not partitioned may be 0. The value of thepartition information of a CU that is partitioned may be 1.

For example, when a single CU is partitioned into four CUs, thehorizontal size and vertical size of each of four CUs generated bypartitioning may be half the horizontal size and the vertical size ofthe CU before being partitioned. When a CU having a 32×32 size ispartitioned into four CUs, the size of each of four partitioned CUs maybe 16×16. When a single CU is partitioned into four CUs, it may beconsidered that the CU has been partitioned in a quad-tree structure.

For example, when a single CU is partitioned into two CUs, thehorizontal size or the vertical size of each of two CUs generated bypartitioning may be half the horizontal size or the vertical size of theCU before being partitioned. When a CU having a 32×32 size is verticallypartitioned into two CUs, the size of each of two partitioned CUs may be16×32. When a CU having a 32×32 size is horizontally partitioned intotwo CUs, the size of each of two partitioned CUs may be 32×16. When asingle CU is partitioned into two CUs, it may be considered that the CUhas been partitioned in a binary-tree structure.

Both of quad-tree partitioning and binary-tree partitioning are appliedto the LCU 310 of FIG. 3.

In the encoding apparatus 100, a Coding Tree Unit (CTU) having a size of64×64 may be partitioned into multiple smaller CUs by a recursivequad-tree structure. A single CU may be partitioned into four CUs havingthe same size. Each CU may be recursively partitioned, and may have aquad-tree structure.

By the recursive partitioning of a CU, an optimal partitioning methodthat incurs a minimum rate-distortion cost may be selected.

FIG. 4 is a diagram illustrating the form of a Prediction Unit (PU) thata Coding Unit (CU) can include.

When, among CUs partitioned from an LCU, a CU, which is not partitionedany further, may be divided into one or more Prediction Units (PUs).Such division is also referred to as “partitioning”.

A PU may be a base unit for prediction. A PU may be encoded and decodedin any one of a skip mode, an inter mode, and an intra mode. A PU may bepartitioned into various shapes depending on respective modes. Forexample, the target block, described above with reference to FIG. 1, andthe target block, described above with reference to FIG. 2, may each bea PU.

A CU may not be split into PUs. When the CU is not split into PUs, thesize of the CU and the size of a PU may be equal to each other.

In a skip mode, partitioning may not be present in a CU. In the skipmode, a 2N×2N mode 410, in which the sizes of a PU and a CU areidentical to each other, may be supported without partitioning.

In an inter mode, 8 types of partition shapes may be present in a CU.For example, in the inter mode, the 2N×2N mode 410, a 2N×N mode 415, anN×2N mode 420, an N×N mode 425, a 2N×nU mode 430, a 2N×nD mode 435, annL×2N mode 440, and an nR×2N mode 445 may be supported.

In an intra mode, the 2N×2N mode 410 and the N×N mode 425 may besupported.

In the 2N×2N mode 410, a PU having a size of 2N×2N may be encoded. ThePU having a size of 2N×2N may mean a PU having a size identical to thatof the CU. For example, the PU having a size of 2N×2N may have a size of64×64, 32×32, 16×16 or 8×8.

In the N×N mode 425, a PU having a size of N×N may be encoded.

For example, in intra prediction, when the size of a PU is 8×8, fourpartitioned PUs may be encoded. The size of each partitioned PU may be4×4.

When a PU is encoded in an intra mode, the PU may be encoded using anyone of multiple intra-prediction modes. For example, HEVC technology mayprovide 35 intra-prediction modes, and the PU may be encoded in any oneof the 35 intra-prediction modes.

Which one of the 2N×2N mode 410 and the N×N mode 425 is to be used toencode the PU may be determined based on rate-distortion cost.

The encoding apparatus 100 may perform an encoding operation on a PUhaving a size of 2N×2N. Here, the encoding operation may be theoperation of encoding the PU in each of multiple intra-prediction modesthat can be used by the encoding apparatus 100. Through the encodingoperation, the optimal intra-prediction mode for a PU having a size of2N×2N may be derived. The optimal intra-prediction mode may be anintra-prediction mode in which a minimum rate-distortion cost occursupon encoding the PU having a size of 2N×2N, among multipleintra-prediction modes that can be used by the encoding apparatus 100.

Further, the encoding apparatus 100 may sequentially perform an encodingoperation on respective PUs obtained from N×N partitioning. Here, theencoding operation may be the operation of encoding a PU in each ofmultiple intra-prediction modes that can be used by the encodingapparatus 100. By means of the encoding operation, the optimalintra-prediction mode for the PU having a size of N×N may be derived.The optimal intra-prediction mode may be an intra-prediction mode inwhich a minimum rate-distortion cost occurs upon encoding the PU havinga size of N×N, among multiple intra-prediction modes that can be used bythe encoding apparatus 100.

The encoding apparatus 100 may determine which of a PU having a size of2N×2N and PUs having sizes of N×N to be encoded based on a comparison ofa rate-distortion cost of the PU having a size of 2N×2N and arate-distortion costs of the PUs having sizes of N×N.

A single CU may be partitioned into one or more PUs, and a PU may bepartitioned into multiple PUs.

For example, when a single PU is partitioned into four PUs, thehorizontal size and vertical size of each of four PUs generated bypartitioning may be half the horizontal size and the vertical size ofthe PU before being partitioned. When a PU having a 32×32 size ispartitioned into four PUs, the size of each of four partitioned PUs maybe 16×16. When a single PU is partitioned into four PUs, it may beconsidered that the PU has been partitioned in a quad-tree structure.

For example, when a single PU is partitioned into two PUs, thehorizontal size or the vertical size of each of two PUs generated bypartitioning may be half the horizontal size or the vertical size of thePU before being partitioned. When a PU having a 32×32 size is verticallypartitioned into two PUs, the size of each of two partitioned PUs may be16×32. When a PU having a 32×32 size is horizontally partitioned intotwo PUs, the size of each of two partitioned PUs may be 32×16. When asingle PU is partitioned into two PUs, it may be considered that the PUhas been partitioned in a binary-tree structure.

FIG. 5 is a diagram illustrating the form of a Transform Unit (TU) thatcan be included in a CU.

A Transform Unit (TU) may have a base unit that is used for a procedure,such as transform, quantization, inverse transform, dequantization,entropy encoding, and entropy decoding, in a CU.

A TU may have a square shape or a rectangular shape. A shape of a TU maybe determined based on a size and/or a shape of a CU.

Among CUs partitioned from the LCU, a CU which is not partitioned intoCUs any further may be partitioned into one or more TUs. Here, thepartition structure of a TU may be a quad-tree structure. For example,as shown in FIG. 5, a single CU 510 may be partitioned one or more timesdepending on the quad-tree structure. By means of this partitioning, thesingle CU 510 may be composed of TUs having various sizes.

It can be considered that when a single CU is split two or more times,the CU is recursively split. Through splitting, a single CU may becomposed of Transform Units (TUs) having various sizes.

Alternatively, a single CU may be split into one or more TUs based onthe number of vertical lines and/or horizontal lines that split the CU.

A CU may be split into symmetric TUs or asymmetric TUs. For splittinginto asymmetric TUs, information about the size and/or shape of each TUmay be signaled from the encoding apparatus 100 to the decodingapparatus 200. Alternatively, the size and/or shape of each TU may bederived from information about the size and/or shape of the CU.

A CU may not be split into TUs. When the CU is not split into TUs, thesize of the CU and the size of a TU may be equal to each other.

A single CU may be partitioned into one or more TUs, and a TU may bepartitioned into multiple TUs.

For example, when a single TU is partitioned into four TUs, thehorizontal size and vertical size of each of four TUs generated bypartitioning may be half the horizontal size and the vertical size ofthe TU before being partitioned. When a TU having a 32×32 size ispartitioned into four TUs, the size of each of four partitioned TUs maybe 16×16. When a single TU is partitioned into four TUs, it may beconsidered that the TU has been partitioned in a quad-tree structure.

For example, when a single TU is partitioned into two TUs, thehorizontal size or the vertical size of each of two TUs generated bypartitioning may be half the horizontal size or the vertical size of theTU before being partitioned. When a TU having a 32×32 size is verticallypartitioned into two TUs, the size of each of two partitioned TUs may be16×32. When a TU having a 32×32 size is horizontally partitioned intotwo TUs, the size of each of two partitioned TUs may be 32×16. When asingle TU is partitioned into two TUs, it may be considered that the TUhas been partitioned in a binary-tree structure.

FIG. 6 illustrates the splitting of a block according to an example.

In a video encoding and/or decoding process, a target block may besplit, as illustrated in FIG. 6.

For splitting of the target block, an indicator indicating splitinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200. The split information may be informationindicating how the target block is split.

The split information may be one or more of a split flag (hereinafterreferred to as “split_flag”), a quad-binary flag (hereinafter referredto as “QB_flag”), a quad-tree flag (hereinafter referred to as“quadtree_flag”), a binary tree flag (hereinafter referred to as“binarytree_flag”), and a binary type flag (hereinafter referred to as“Btype_flag”).

“split_flag” may be a flag indicating whether a block is split. Forexample, a split_flag value of 1 may indicate that the correspondingblock is split. A split_flag value of 0 may indicate that thecorresponding block is not split.

“QB_flag” may be a flag indicating which one of a quad-tree form and abinary tree form corresponds to the shape in which the block is split.For example, a QB_flag value of 0 may indicate that the block is splitin a quad-tree form. A QB_flag value of 1 may indicate that the block issplit in a binary tree form. Alternatively, a QB_flag value of 0 mayindicate that the block is split in a binary tree form. A QB_flag valueof 1 may indicate that the block is split in a quad-tree form.

“quadtree_flag” may be a flag indicating whether a block is split in aquad-tree form. For example, a quadtree_flag value of 1 may indicatethat the block is split in a quad-tree form. A quadtree_flag value of 0may indicate that the block is not split in a quad-tree form.

“binarytree_flag” may be a flag indicating whether a block is split in abinary tree form. For example, a binarytree_flag value of 1 may indicatethat the block is split in a binary tree form. A binarytree_flag valueof 0 may indicate that the block is not split in a binary tree form.

“Btype_flag” may be a flag indicating which one of a vertical split anda horizontal split corresponds to a split direction when a block issplit in a binary tree form. For example, a Btype_flag value of 0 mayindicate that the block is split in a horizontal direction. A Btype_flagvalue of 1 may indicate that a block is split in a vertical direction.Alternatively, a Btype_flag value of 0 may indicate that the block issplit in a vertical direction. A Btype_flag value of 1 may indicate thata block is split in a horizontal direction.

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of quadtree_flag, binarytree_flag, andBtype_flag, as shown in the following Table 1.

TABLE 1 quadtree_flag binarytree_flag Btype_flag 1 0 1 1 0 0 1 0 1 0 0 00 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of split_flag, QB_flag and Btype_flag, asshown in the following Table 2.

TABLE 2 split_flag QB_flag Btype_flag 1 0 1 1 1 0 0 1 0 1 1 0 0 0 0 0 01 1 0 1 1 0 0 0 0

The splitting method may be limited only to a quad-tree or to a binarytree depending on the size and/or shape of the block. When thislimitation is applied, split_flag may be a flag indicating whether ablock is split in a quad-tree form or a flag indicating whether a blockis split in a binary tree form. The size and shape of a block may bederived depending on the depth information of the block, and the depthinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

When the size of a block falls within a specific range, only splittingin a quad-tree form may be possible. For example, the specific range maybe defined by at least one of a maximum block size and a minimum blocksize at which only splitting in a quad-tree form is possible.

Information indicating the maximum block size and the minimum block sizeat which only splitting in a quad-tree form is possible may be signaledfrom the encoding apparatus 100 to the decoding apparatus 200 through abitstream. Further, this information may be signaled for at least one ofunits such as a video, a sequence, a picture, and a slice (or asegment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 64×64 andbelow 256×256, only splitting in a quad-tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in aquad-tree form is performed.

When the size of a block falls within the specific range, only splittingin a binary tree form may be possible. For example, the specific rangemay be defined by at least one of a maximum block size and a minimumblock size at which only splitting in a binary tree form is possible.

Information indicating the maximum block size and/or the minimum blocksize at which only splitting in a binary tree form is possible may besignaled from the encoding apparatus 100 to the decoding apparatus 200through a bitstream. Further, this information may be signaled for atleast one of units such as a sequence, a picture, and a slice (or asegment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 8×8 andbelow 16×16, only splitting in a binary tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in abinary tree form is performed.

The splitting of a block may be limited by previous splitting. Forexample, when a block is split in a binary tree form and multiplepartition blocks are generated, each partition block may be additionallysplit only in a binary tree form.

When the horizontal size or vertical size of a partition block is a sizethat cannot be split further, the above-described indicator may not besignaled.

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocess.

Arrows radially extending from the center of the graph in FIG. 7indicate the prediction directions of intra-prediction modes. Further,numbers appearing near the arrows indicate examples of mode valuesassigned to intra-prediction modes or to the prediction directions ofthe intra-prediction modes.

Intra encoding and/or decoding may be performed using reference samplesof blocks neighboring a target block. The neighboring blocks may beneighboring reconstructed blocks. For example, intra encoding and/ordecoding may be performed using the values of reference samples whichare included in each neighboring reconstructed block or the codingparameters of the neighboring reconstructed block.

The encoding apparatus 100 and/or the decoding apparatus 200 maygenerate a prediction block by performing intra prediction on a targetblock based on information about samples in a target image. When intraprediction is performed, the encoding apparatus 100 and/or the decodingapparatus 200 may generate a prediction block for the target block byperforming intra prediction based on information about samples in thetarget image. When intra prediction is performed, the encoding apparatus100 and/or the decoding apparatus 200 may perform directional predictionand/or non-directional prediction based on at least one reconstructedreference sample.

A prediction block may be a block generated as a result of performingintra prediction. A prediction block may correspond to at least one of aCU, a PU, and a TU.

The unit of a prediction block may have a size corresponding to at leastone of a CU, a PU, and a TU. The prediction block may have a squareshape having a size of 2N×2N or N×N. The size of N×N may include sizesof 4×4, 8×8, 16×16, 32×32, 64×64, or the like.

Alternatively, a prediction block may a square block having a size of2×2, 4×4, 8×8, 16×16, 32×32, 64×64 or the like or a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16, 8×16, or the like.

Intra prediction may be performed in consideration of theintra-prediction mode for the target block. The number ofintra-prediction modes that the target block can have may be apredefined fixed value, and may be a value determined differentlydepending on the attributes of a prediction block. For example, theattributes of the prediction block may include the size of theprediction block, the type of prediction block, etc.

For example, the number of intra-prediction modes may be fixed at 35regardless of the size of a prediction block. Alternatively, the numberof intra-prediction modes may be, for example, 3, 5, 9, 17, 34, 35, or36.

The intra-prediction modes may be non-directional modes or directionalmodes. For example, the intra-prediction modes may include twonon-directional modes and 33 directional modes, as shown in FIG. 6.

The two non-directional modes may include a DC mode and a planar mode.

The directional modes may be prediction modes having a specificdirection or a specific angle.

The intra-prediction modes may each be represented by at least one of amode number, a mode value, and a mode angle. The number ofintra-prediction modes may be M. The value of M may be 1 or more. Inother words, the number of intra-prediction modes may be M, whichincludes the number of non-directional modes and the number ofdirectional modes.

The number of intra-prediction modes may be fixed to M regardless of thesize of a block. For example, the number of intra-prediction modes maybe fixed at any one of 35 and 67 regardless of the size of a block.

Alternatively, the number of intra-prediction modes may differ dependingon the size of a block and/or the type of color component.

For example, the larger the size of the block, the greater the number ofintra-prediction modes. Alternatively, the larger the size of the block,the smaller the number of intra-prediction modes. When the size of theblock is 4×4 or 8×8, the number of intra-prediction modes may be 67.When the size of the block is 16×16, the number of intra-predictionmodes may be 35. When the size of the block is 32×32, the number ofintra-prediction modes may be 19. When the size of a block is 64×64, thenumber of intra-prediction modes may be 7.

For example, the number of intra prediction modes may differ dependingon whether a color component is a luma signal or a chroma signal.Alternatively, the number of intra-prediction modes corresponding to aluma component block may be greater than the number of intra-predictionmodes corresponding to a chroma component block.

For example, in a vertical mode having a mode value of 26, predictionmay be performed in a vertical direction based on the pixel value of areference sample. For example, in a horizontal mode having a mode valueof 10, prediction may be performed in a horizontal direction based onthe pixel value of a reference sample.

Even in directional modes other than the above-described mode, theencoding apparatus 100 and the decoding apparatus 200 may perform intraprediction on a target unit using reference samples depending on anglescorresponding to the directional modes.

Intra-prediction modes located on a right side with respect to thevertical mode may be referred to as ‘vertical-right modes’.Intra-prediction modes located below the horizontal mode may be referredto as ‘horizontal-below modes’. For example, in FIG. 6, theintra-prediction modes in which a mode value is one of 27, 28, 29, 30,31, 32, 33, and 34 may be vertical-right modes 613. Intra-predictionmodes in which a mode value is one of 2, 3, 4, 5, 6, 7, 8, and 9 may behorizontal-below modes 616.

The non-directional mode may include a DC mode and a planar mode. Forexample, a value of the DC mode may be 1. A value of the planar mode maybe 0.

The directional mode may include an angular mode. Among the plurality ofthe intra prediction modes, remaining modes except for the DC mode andthe planar mode may be directional modes.

When the intra-prediction mode is a DC mode, a prediction block may begenerated based on the average of pixel values of a plurality ofreference pixels. For example, a value of a pixel of a prediction blockmay be determined based on the average of pixel values of a plurality ofreference pixels.

The number of above-described intra-prediction modes and the mode valuesof respective intra-prediction modes are merely exemplary. The number ofabove-described intra-prediction modes and the mode values of respectiveintra-prediction modes may be defined differently depending on theembodiments, implementation and/or requirements.

In order to perform intra prediction on a target block, the step ofchecking whether samples included in a reconstructed neighboring blockcan be used as reference samples of a target block may be performed.When a sample that cannot be used as a reference sample of the targetblock is present among samples in the neighboring block, a valuegenerated via copying and/or interpolation that uses at least one samplevalue, among the samples included in the reconstructed neighboringblock, may replace the sample value of the sample that cannot be used asthe reference sample. When the value generated via copying and/orinterpolation replaces the sample value of the existing sample, thesample may be used as the reference sample of the target block.

In intra prediction, a filter may be applied to at least one of areference sample and a prediction sample based on at least one of theintra-prediction mode and the size of the target block.

The type of filter to be applied to at least one of a reference sampleand a prediction sample may differ depending on at least one of theintra-prediction mode of a target block, the size of the target block,and the shape of the target block. The types of filters may beclassified depending on one or more of the number of filter taps, thevalue of a filter coefficient, and filter strength.

When the intra-prediction mode is a planar mode, a sample value of aprediction target block may be generated using a weighted sum of anabove reference sample of the target block, a left reference sample ofthe target block, an above-right reference sample of the target block,and a below-left reference sample of the target block depending on thelocation of the prediction target sample in the prediction block whenthe prediction block of the target block is generated.

When the intra-prediction mode is a DC mode, the average of referencesamples above the target block and the reference samples to the left ofthe target block may be used when the prediction block of the targetblock is generated. Also, filtering using the values of referencesamples may be performed on specific rows or specific columns in thetarget block. The specific rows may be one or more upper rows adjacentto the reference sample. The specific columns may be one or more leftcolumns adjacent to the reference sample.

When the intra-prediction mode is a directional mode, a prediction blockmay be generated using the above reference samples, left referencesamples, above-right reference sample and/or below-left reference sampleof the target block.

In order to generate the above-described prediction sample,real-number-based interpolation may be performed.

The intra-prediction mode of the target block may be predicted fromintra prediction mode of a neighboring block adjacent to the targetblock, and the information used for prediction may beentropy-encoded/decoded.

For example, when the intra-prediction modes of the target block and theneighboring block are identical to each other, it may be signaled, usinga predefined flag, that the intra-prediction modes of the target blockand the neighboring block are identical.

For example, an indicator for indicating an intra-prediction modeidentical to that of the target block, among intra-prediction modes ofmultiple neighboring blocks, may be signaled.

When the intra-prediction modes of the target block and a neighboringblock are different from each other, information about theintra-prediction mode of the target block may be encoded and/or decodedusing entropy encoding and/or decoding.

FIG. 8 is a diagram for explaining the locations of reference samplesused in an intra-prediction procedure.

FIG. 8 illustrates the locations of reference samples used for intraprediction of a target block. Referring to FIG. 8, reconstructedreference samples used for intra prediction of the target block mayinclude below-left reference samples 831, left reference samples 833, anabove-left corner reference sample 835, above reference samples 837, andabove-right reference samples 839.

For example, the left reference samples 833 may mean reconstructedreference pixels adjacent to the left side of the target block. Theabove reference samples 837 may mean reconstructed reference pixelsadjacent to the top of the target block. The above-left corner referencesample 835 may mean a reconstructed reference pixel located at theabove-left corner of the target block. The below-left reference samples831 may mean reference samples located below a left sample line composedof the left reference samples 833, among samples located on the sameline as the left sample line. The above-right reference samples 839 maymean reference samples located to the right of an above sample linecomposed of the above reference samples 837, among samples located onthe same line as the above sample line.

When the size of a target block is N×N, the numbers of the below-leftreference samples 831, the left reference samples 833, the abovereference samples 837, and the above-right reference samples 839 mayeach be N.

By performing intra prediction on the target block, a prediction blockmay be generated. The generation of the prediction block may include thedetermination of the values of pixels in the prediction block. The sizesof the target block and the prediction block may be equal.

The reference samples used for intra prediction of the target block mayvary depending on the intra-prediction mode of the target block. Thedirection of the intra-prediction mode may represent a dependencerelationship between the reference samples and the pixels of theprediction block. For example, the value of a specified reference samplemay be used as the values of one or more specified pixels in theprediction block. In this case, the specified reference sample and theone or more specified pixels in the prediction block may be the sampleand pixels which are positioned in a straight line in the direction ofan intra-prediction mode. In other words, the value of the specifiedreference sample may be copied as the value of a pixel located in adirection reverse to the direction of the intra-prediction mode.Alternatively, the value of a pixel in the prediction block may be thevalue of a reference sample located in the direction of theintra-prediction mode with respect to the location of the pixel.

In an example, when the intra-prediction mode of a target block is avertical mode having a mode value of 26, the above reference samples 837may be used for intra prediction. When the intra-prediction mode is thevertical mode, the value of a pixel in the prediction block may be thevalue of a reference sample vertically located above the location of thepixel. Therefore, the above reference samples 837 adjacent to the top ofthe target block may be used for intra prediction. Furthermore, thevalues of pixels in one row of the prediction block may be identical tothose of the above reference samples 837.

In an example, when the intra-prediction mode of a target block is ahorizontal mode having a mode value of 10, the left reference samples833 may be used for intra prediction. When the intra-prediction mode isthe horizontal mode, the value of a pixel in the prediction block may bethe value of a reference sample horizontally located left to thelocation of the pixel. Therefore, the left reference samples 833adjacent to the left of the target block may be used for intraprediction. Furthermore, the values of pixels in one column of theprediction block may be identical to those of the left reference samples833.

In an example, when the mode value of the intra-prediction mode of thecurrent block is 18, at least some of the left reference samples 833,the above-left corner reference sample 835, and at least some of theabove reference samples 837 may be used for intra prediction. When themode value of the intra-prediction mode is 18, the value of a pixel inthe prediction block may be the value of a reference sample diagonallylocated at the above-left corner of the pixel.

Further, At least a part of the above-right reference samples 839 may beused for intra prediction in a case that a intra prediction mode havinga mode value of 27, 28, 29, 30, 31, 32, 33 or 34 is used.

Further, At least a part of the below-left reference samples 831 may beused for intra prediction in a case that a intra prediction mode havinga mode value of 2, 3, 4, 5, 6, 7, 8 or 9 is used.

Further, the above-left corner reference sample 835 may be used forintra prediction in a case that a intra prediction mode of which a modevalue is a value ranging from 11 to 25.

The number of reference samples used to determine the pixel value of onepixel in the prediction block may be either 1, or 2 or more.

As described above, the pixel value of a pixel in the prediction blockmay be determined depending on the location of the pixel and thelocation of a reference sample indicated by the direction of theintra-prediction mode. When the location of the pixel and the locationof the reference sample indicated by the direction of theintra-prediction mode are integer positions, the value of one referencesample indicated by an integer position may be used to determine thepixel value of the pixel in the prediction block.

When the location of the pixel and the location of the reference sampleindicated by the direction of the intra-prediction mode are not integerpositions, an interpolated reference sample based on two referencesamples closest to the location of the reference sample may begenerated. The value of the interpolated reference sample may be used todetermine the pixel value of the pixel in the prediction block. In otherwords, when the location of the pixel in the prediction block and thelocation of the reference sample indicated by the direction of theintra-prediction mode indicate the location between two referencesamples, an interpolated value based on the values of the two samplesmay be generated.

The prediction block generated via prediction may not be identical to anoriginal target block. In other words, there may be a prediction errorwhich is the difference between the target block and the predictionblock, and there may also be a prediction error between the pixel of thetarget block and the pixel of the prediction block.

Hereinafter, the terms “difference”, “error”, and “residual” may be usedto have the same meaning, and may be used interchangeably with eachother.

For example, in the case of directional intra prediction, the longer thedistance between the pixel of the prediction block and the referencesample, the greater the prediction error that may occur. Such aprediction error may result in discontinuity between the generatedprediction block and neighboring blocks.

In order to reduce the prediction error, filtering for the predictionblock may be used. Filtering may be configured to adaptively apply afilter to an area, regarded as having a large prediction error, in theprediction block. For example, the area regarded as having a largeprediction error may be the boundary of the prediction block. Further,an area regarded as having a large prediction error in the predictionblock may differ depending on the intra-prediction mode, and thecharacteristics of filters may also differ depending thereon.

FIG. 9 is a diagram for explaining an embodiment of an inter predictionprocedure.

The rectangles shown in FIG. 9 may represent images (or pictures).Further, in FIG. 9, arrows may represent prediction directions. That is,each image may be encoded and/or decoded depending on the predictiondirection.

Images may be classified into an Intra Picture (I picture), aUni-prediction Picture or Predictive Coded Picture (P picture), and aBi-prediction Picture or Bi-predictive Coded Picture (B picture)depending on the encoding type. Each picture may be encoded and/ordecoded depending on the encoding type thereof.

When a target image that is the target to be encoded is an I picture,the target image may be encoded using data contained in the image itselfwithout inter prediction that refers to other images. For example, an Ipicture may be encoded only via intra prediction.

When a target image is a P picture, the target image may be encoded viainter prediction, which uses reference pictures existing in onedirection. Here, the one direction may be a forward direction or abackward direction.

When a target image is a B picture, the image may be encoded via interprediction that uses reference pictures existing in two directions, ormay be encoded via inter prediction that uses reference picturesexisting in one of a forward direction and a backward direction. Here,the two directions may be the forward direction and the backwarddirection.

A P picture and a B picture that are encoded and/or decoded usingreference pictures may be regarded as images in which inter predictionis used.

Below, inter prediction in an inter mode according to an embodiment willbe described in detail.

Inter prediction may be performed using motion information.

In an inter mode, the encoding apparatus 100 may perform interprediction and/or motion compensation on a target block. The decodingapparatus 200 may perform inter prediction and/or motion compensation,corresponding to inter prediction and/or motion compensation performedby the encoding apparatus 100, on a target block.

Motion information of the target block may be individually derived bythe encoding apparatus 100 and the decoding apparatus 200 during theinter prediction. The motion information may be derived using motioninformation of a reconstructed neighboring block, motion information ofa col block, and/or motion information of a block adjacent to the colblock.

For example, the encoding apparatus 100 or the decoding apparatus 200may perform prediction and/or motion compensation by using motioninformation of a spatial candidate and/or a temporal candidate as motioninformation of the target block. The target block may mean a PU and/or aPU partition.

A spatial candidate may be a reconstructed block which is spatiallyadjacent to the target block.

A temporal candidate may be a reconstructed block corresponding to thetarget block in a previously reconstructed co-located picture (colpicture).

In inter prediction, the encoding apparatus 100 and the decodingapparatus 200 may improve encoding efficiency and decoding efficiency byutilizing the motion information of a spatial candidate and/or atemporal candidate. The motion information of a spatial candidate may bereferred to as ‘spatial motion information’. The motion information of atemporal candidate may be referred to as ‘temporal motion information’.

Below, the motion information of a spatial candidate may be the motioninformation of a PU including the spatial candidate. The motioninformation of a temporal candidate may be the motion information of aPU including the temporal candidate. The motion information of acandidate block may be the motion information of a PU including thecandidate block.

Inter prediction may be performed using a reference picture.

The reference picture may be at least one of a picture previous to atarget picture and a picture subsequent to the target picture. Thereference picture may be an image used for the prediction of the targetblock.

In inter prediction, a region in the reference picture may be specifiedby utilizing a reference picture index (or refIdx) for indicating areference picture, a motion vector, which will be described later, etc.Here, the region specified in the reference picture may indicate areference block.

Inter prediction may select a reference picture, and may also select areference block corresponding to the target block from the referencepicture. Further, inter prediction may generate a prediction block forthe target block using the selected reference block.

The motion information may be derived during inter prediction by each ofthe encoding apparatus 100 and the decoding apparatus 200.

A spatial candidate may be a block 1) which is present in a targetpicture, 2) which has been previously reconstructed via encoding and/ordecoding, and 3) which is adjacent to the target block or is located atthe corner of the target block. Here, the “block located at the cornerof the target block” may be either a block vertically adjacent to aneighboring block that is horizontally adjacent to the target block, ora block horizontally adjacent to a neighboring block that is verticallyadjacent to the target block. Further, “block located at the corner ofthe target block” may have the same meaning as “block adjacent to thecorner of the target block”. The meaning of “block located at the cornerof the target block” may be included in the meaning of “block adjacentto the target block”.

For example, a spatial candidate may be a reconstructed block located tothe left of the target block, a reconstructed block located above thetarget block, a reconstructed block located at the below-left corner ofthe target block, a reconstructed block located at the above-rightcorner of the target block, or a reconstructed block located at theabove-left corner of the target block.

Each of the encoding apparatus 100 and the decoding apparatus 200 mayidentify a block present at the location spatially corresponding to thetarget block in a col picture. The location of the target block in thetarget picture and the location of the identified block in the colpicture may correspond to each other.

Each of the encoding apparatus 100 and the decoding apparatus 200 maydetermine a col block present at the predefined relative location forthe identified block to be a temporal candidate. The predefined relativelocation may be a location present inside and/or outside the identifiedblock.

For example, the col block may include a first col block and a secondcol block. When the coordinates of the identified block are (xP, yP) andthe size of the identified block is represented by (nPSW, nPSH), thefirst col block may be a block located at coordinates (xP+nPSW,yP+nPSH). The second col block may be a block located at coordinates(xP+(nPSW>>1), yP+(nPSH>>1)). The second col block may be selectivelyused when the first col block is unavailable.

The motion vector of the target block may be determined based on themotion vector of the col block. Each of the encoding apparatus 100 andthe decoding apparatus 200 may scale the motion vector of the col block.The scaled motion vector of the col block may be used as the motionvector of the target block. Further, a motion vector for the motioninformation of a temporal candidate stored in a list may be a scaledmotion vector.

The ratio of the motion vector of the target block to the motion vectorof the col block may be identical to the ratio of a first distance to asecond distance. The first distance may be the distance between thereference picture and the target picture of the target block. The seconddistance may be the distance between the reference picture and the colpicture of the col block.

The scheme for deriving motion information may change depending on theinter-prediction mode of a target block. For example, asinter-prediction modes applied for inter prediction, an Advanced MotionVector Predictor (AMVP) mode, a merge mode, a skip mode, a currentpicture reference mode, etc. may be present. The merge mode may also bereferred to as a “motion merge mode”. Individual modes will be describedin detail below.

1) AMVP Mode

When an AMVP mode is used, the encoding apparatus 100 may search aneighboring region of a target block for a similar block. The encodingapparatus 100 may acquire a prediction block by performing prediction onthe target block using motion information of the found similar block.The encoding apparatus 100 may encode a residual block, which is thedifference between the target block and the prediction block.

1-1) Creation of List of Prediction Motion Vector Candidates

When an AMVP mode is used as the prediction mode, each of the encodingapparatus 100 and the decoding apparatus 200 may create a list ofprediction motion vector candidates using the motion vector of a spatialcandidate, the motion vector of a temporal candidate, and a zero vector.The prediction motion vector candidate list may include one or moreprediction motion vector candidates. At least one of the motion vectorof a spatial candidate, the motion vector of a temporal candidate, and azero vector may be determined and used as a prediction motion vectorcandidate.

Hereinafter, the terms “prediction motion vector (candidate)” and“motion vector (candidate)” may be used to have the same meaning, andmay be used interchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate” and “AMVPcandidate” may be used to have the same meaning, and may be usedinterchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate list” and“AMVP candidate list” may be used to have the same meaning, and may beused interchangeably with each other.

Spatial candidates may include a reconstructed spatial neighboringblock. In other words, the motion vector of the reconstructedneighboring block may be referred to as a “spatial prediction motionvector candidate”.

Temporal candidates may include a col block and a block adjacent to thecol block. In other words, the motion vector of the col block or themotion vector of the block adjacent to the col block may be referred toas a “temporal prediction motion vector candidate”.

The zero vector may be a (0, 0) motion vector.

The prediction motion vector candidates may be motion vector predictorsfor predicting a motion vector. Also, in the encoding apparatus 100,each prediction motion vector candidate may be an initial searchlocation for a motion vector.

1-2) Search for Motion Vectors that Use List of Prediction Motion VectorCandidates

The encoding apparatus 100 may determine the motion vector to be used toencode a target block within a search range using a list of predictionmotion vector candidates. Further, the encoding apparatus 100 maydetermine a prediction motion vector candidate to be used as theprediction motion vector of the target block, among prediction motionvector candidates present in the prediction motion vector candidatelist.

The motion vector to be used to encode the target block may be a motionvector that can be encoded at minimum cost.

Further, the encoding apparatus 100 may determine whether to use theAMVP mode to encode the target block.

1-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream includinginter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether an AMVP mode is used, 2) a prediction motion vectorindex, 3) a Motion Vector Difference (MVD), 4) a reference direction,and 5) a reference picture index.

Hereinafter, the terms “prediction motion vector index” and “AMVP index”may be used to have the same meaning, and may be used interchangeablywith each other.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire a prediction motion vector index,an MVD, a reference direction, and a reference picture index from thebitstream through entropy decoding when mode information indicates thatthe AMVP mode is used.

The prediction motion vector index may indicate a prediction motionvector candidate to be used for the prediction of a target block, amongprediction motion vector candidates included in the prediction motionvector candidate list.

1-4) Inter Prediction in AMVP Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may derive prediction motion vectorcandidates using a prediction motion vector candidate list, and maydetermine the motion information of a target block based on the derivedprediction motion vector candidates.

The decoding apparatus 200 may determine a motion vector candidate forthe target block, among the prediction motion vector candidates includedin the prediction motion vector candidate list, using a predictionmotion vector index. The decoding apparatus 200 may select a predictionmotion vector candidate, indicated by the prediction motion vectorindex, from among prediction motion vector candidates included in theprediction motion vector candidate list, as the prediction motion vectorof the target block.

The motion vector to be actually used for inter prediction of the targetblock may not match the prediction motion vector. In order to indicatethe difference between the motion vector to be actually used for interprediction of the target block and the prediction motion vector, an MVDmay be used. The encoding apparatus 100 may derive a prediction motionvector similar to the motion vector to be actually used for interprediction of the target block so as to use an MVD that is as small aspossible.

An MVD may be the difference between the motion vector of the targetblock and the prediction motion vector. The encoding apparatus 100 maycalculate an MVD and may entropy-encode the MVD.

The MVD may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200 through a bitstream. The decoding apparatus 200may decode the received MVD. The decoding apparatus 200 may derive themotion vector of the target block by summing the decoded MVD and theprediction motion vector. In other words, the motion vector of thetarget block derived by the decoding apparatus 200 may be the sum of theentropy-decoded MVD and the motion vector candidate.

The reference direction may indicate a list of reference pictures to beused for prediction of the target block. For example, the referencedirection may indicate one of a reference picture list L0 and areference picture list L1.

The reference direction merely indicates the reference picture list tobe used for prediction of the target block, and may not mean that thedirections of reference pictures are limited to a forward direction or abackward direction. In other words, each of the reference picture listL0 and the reference picture list L1 may include pictures in a forwarddirection and/or a backward direction.

That the reference direction is unidirectional may mean that a singlereference picture list is used. That the reference direction isbidirectional may mean that two reference picture lists are used. Inother words, the reference direction may indicate one of the case whereonly the reference picture list L0 is used, the case where only thereference picture list L1 is used, and the case where two referencepicture lists are used.

The reference picture index may indicate a reference picture to be usedfor prediction of a target block, among reference pictures in thereference picture list. The reference picture index may beentropy-encoded by the encoding apparatus 100. The entropy-encodedreference picture index may be signaled to the decoding apparatus 200 bythe encoding apparatus 100 through a bitstream.

When two reference picture lists are used to predict the target block, asingle reference picture index and a single motion vector may be usedfor each of the reference picture lists. Further, when two referencepicture lists are used to predict the target block, two predictionblocks may be specified for the target block. For example, the (final)prediction block of the target block may be generated using the averageor weighted sum of the two prediction blocks for the target block.

The motion vector of the target block may be derived by the predictionmotion vector index, the MVD, the reference direction, and the referencepicture index.

The decoding apparatus 200 may generate a prediction block for thetarget block based on the derived motion vector and the referencepicture index. For example, the prediction block may be a referenceblock, indicated by the derived motion vector, in the reference pictureindicated by the reference picture index.

Since the prediction motion vector index and the MVD are encoded withoutthe motion vector itself of the target block being encoded, the numberof bits transmitted from the encoding apparatus 100 to the decodingapparatus 200 may be decreased, and encoding efficiency may be improved.

For the target block, the motion information of reconstructedneighboring blocks may be used. In a specific inter-prediction mode, theencoding apparatus 100 may not separately encode the actual motioninformation of the target block. The motion information of the targetblock is not encoded, and additional information that enables the motioninformation of the target block to be derived using the motioninformation of reconstructed neighboring blocks may be encoded instead.As the additional information is encoded, the number of bits transmittedto the decoding apparatus 200 may be decreased, and encoding efficiencymay be improved.

For example, as inter-prediction modes in which the motion informationof the target block is not directly encoded, there may be a skip modeand/or a merge mode. Here, each of the encoding apparatus 100 and thedecoding apparatus 200 may use an identifier and/or an index thatindicates a unit, the motion information of which is to be used as themotion information of the target unit, among reconstructed neighboringunits.

2) Merge Mode

As a scheme for deriving the motion information of a target block, thereis merging. The term “merging” may mean the merging of the motion ofmultiple blocks. “Merging” may mean that the motion information of oneblock is also applied to other blocks. In other words, a merge mode maybe a mode in which the motion information of the target block is derivedfrom the motion information of a neighboring block.

When a merge mode is used, the encoding apparatus 100 may predict themotion information of a target block using the motion information of aspatial candidate and/or the motion information of a temporal candidate.The spatial candidate may include a reconstructed spatial neighboringblock that is spatially adjacent to the target block. The spatialneighboring block may include a left adjacent block and an aboveadjacent block. The temporal candidate may include a col block. Theterms “spatial candidate” and “spatial merge candidate” may be used tohave the same meaning, and may be used interchangeably with each other.The terms “temporal candidate” and “temporal merge candidate” may beused to have the same meaning, and may be used interchangeably with eachother.

The encoding apparatus 100 may acquire a prediction block viaprediction. The encoding apparatus 100 may encode a residual block,which is the difference between the target block and the predictionblock.

2-1) Creation of Merge Candidate List

When the merge mode is used, each of the encoding apparatus 100 and thedecoding apparatus 200 may create a merge candidate list using themotion information of a spatial candidate and/or the motion informationof a temporal candidate. The motion information may include 1) a motionvector, 2) a reference picture index, and 3) a reference direction. Thereference direction may be unidirectional or bidirectional.

The merge candidate list may include merge candidates. The mergecandidates may be motion information. In other words, the mergecandidate list may be a list in which pieces of motion information arestored.

The merge candidates may be pieces of motion information of temporalcandidates and/or spatial candidates. Further, the merge candidate listmay include new merge candidates generated by a combination of mergecandidates that are already present in the merge candidate list. Inother words, the merge candidate list may include new motion informationgenerated by a combination of pieces of motion information previouslypresent in the merge candidate list.

The merge candidates may be specific modes deriving inter predictioninformation. The merge candidate may be information indicating aspecific mode deriving inter prediction information. Inter predictioninformation of a target block may be derived according to a specificmode which the merge candidate indicates. Furthermore, the specific modemay include a process of deriving a series of inter predictioninformation. This specific mode may be an inter prediction informationderivation mode or a motion information derivation mode.

The inter prediction information of the target block may be derivedaccording to the mode indicated by the merge candidate selected by themerge index among the merge candidates in the merge candidate list

For example, the motion information derivation modes in the mergecandidate list may be at least one of 1) motion information derivationmode for a sub-block unit and 2) an affine motion information derivationmode.

Furthermore, the merge candidate list may include motion information ofa zero vector. The zero vector may also be referred to as a “zero-mergecandidate”.

In other words, pieces of motion information in the merge candidate listmay be at least one of 1) motion information of a spatial candidate, 2)motion information of a temporal candidate, 3) motion informationgenerated by a combination of pieces of motion information previouslypresent in the merge candidate list, and 4) a zero vector.

Motion information may include 1) a motion vector, 2) a referencepicture index, and 3) a reference direction. The reference direction mayalso be referred to as an “inter-prediction indicator”. The referencedirection may be unidirectional or bidirectional. The unidirectionalreference direction may indicate L0 prediction or L1 prediction.

The merge candidate list may be created before prediction in the mergemode is performed.

The number of merge candidates in the merge candidate list may bepredefined. Each of the encoding apparatus 100 and the decodingapparatus 200 may add merge candidates to the merge candidate listdepending on the predefined scheme and predefined priorities so that themerge candidate list has a predefined number of merge candidates. Themerge candidate list of the encoding apparatus 100 and the mergecandidate list of the decoding apparatus 200 may be made identical toeach other using the predefined scheme and the predefined priorities.

Merging may be applied on a CU basis or a PU basis. When merging isperformed on a CU basis or a PU basis, the encoding apparatus 100 maytransmit a bitstream including predefined information to the decodingapparatus 200. For example, the predefined information may contain 1)information indicating whether to perform merging for individual blockpartitions, and 2) information about a block with which merging is to beperformed, among blocks that are spatial candidates and/or temporalcandidates for the target block.

2-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine merge candidates to be used toencode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using merge candidates in themerge candidate list, and may generate residual blocks for the mergecandidates. The encoding apparatus 100 may use a merge candidate thatincurs the minimum cost in prediction and in the encoding of residualblocks to encode the target block.

Further, the encoding apparatus 100 may determine whether to use a mergemode to encode the target block.

2-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The encodingapparatus 100 may generate entropy-encoded inter-prediction informationby performing entropy encoding on inter-prediction information, and maytransmit a bitstream including the entropy-encoded inter-predictioninformation to the decoding apparatus 200. Through the bitstream, theentropy-encoded inter-prediction information may be signaled to thedecoding apparatus 200 by the encoding apparatus 100.

The decoding apparatus 200 may perform inter prediction on the targetblock using the inter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether a merge mode is used and 2) a merge index.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire the merge index from thebitstream only when the mode information indicates that the merge modeis used.

The mode information may be a merge flag. The unit of the modeinformation may be a block. Information about the block may include modeinformation, and the mode information may indicate whether a merge modeis applied to the block.

The merge index may indicate a merge candidate to be used for theprediction of the target block, among merge candidates included in themerge candidate list. Alternatively, the merge index may indicate ablock with which the target block is to be merged, among neighboringblocks spatially or temporally adjacent to the target block.

The encoding apparatus 100 may select a merge candidate having thehighest encoding performance among the merge candidates included in themerge candidate list and set a value of the merge index to indicate theselected merge candidate.

2-4) Inter Prediction of Merge Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing the merge candidate indicated by the merge index, among mergecandidates included in the merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the merge index.

3) Skip Mode

A skip mode may be a mode in which the motion information of a spatialcandidate or the motion information of a temporal candidate is appliedto the target block without change. Also, the skip mode may be a mode inwhich a residual signal is not used. In other words, when the skip modeis used, a reconstructed block may be a prediction block.

The difference between the merge mode and the skip mode lies in whetheror not a residual signal is transmitted or used. That is, the skip modemay be similar to the merge mode except that a residual signal is nottransmitted or used.

When the skip mode is used, the encoding apparatus 100 may transmitinformation about a block, the motion information of which is to be usedas the motion information of the target block, among blocks that arespatial candidates or temporal candidates, to the decoding apparatus 200through a bitstream. The encoding apparatus 100 may generateentropy-encoded information by performing entropy encoding on theinformation, and may signal the entropy-encoded information to thedecoding apparatus 200 through a bitstream.

Further, when the skip mode is used, the encoding apparatus 100 may nottransmit other syntax information, such as an MVD, to the decodingapparatus 200. For example, when the skip mode is used, the encodingapparatus 100 may not signal a syntax element related to at least one ofan MVC, a coded block flag, and a transform coefficient level to thedecoding apparatus 200.

3-1) Creation of Merge Candidate List

The skip mode may also use a merge candidate list. In other words, amerge candidate list may be used both in the merge mode and in the skipmode. In this aspect, the merge candidate list may also be referred toas a “skip candidate list” or a “merge/skip candidate list”.

Alternatively, the skip mode may use an additional candidate listdifferent from that of the merge mode. In this case, in the followingdescription, a merge candidate list and a merge candidate may bereplaced with a skip candidate list and a skip candidate, respectively.

The merge candidate list may be created before prediction in the skipmode is performed.

3-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine the merge candidates to be usedto encode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using the merge candidates in amerge candidate list. The encoding apparatus 100 may use a mergecandidate that incurs the minimum cost in prediction to encode thetarget block.

Further, the encoding apparatus 100 may determine whether to use a skipmode to encode the target block.

3-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may include 1) mode informationindicating whether a skip mode is used, and 2) a skip index.

The skip index may be identical to the above-described merge index.

When the skip mode is used, the target block may be encoded withoutusing a residual signal. The inter-prediction information may notcontain a residual signal. Alternatively, the bitstream may not includea residual signal.

The decoding apparatus 200 may acquire a skip index from the bitstreamonly when the mode information indicates that the skip mode is used. Asdescribed above, a merge index and a skip index may be identical to eachother. The decoding apparatus 200 may acquire the skip index from thebitstream only when the mode information indicates that the merge modeor the skip mode is used.

The skip index may indicate the merge candidate to be used for theprediction of the target block, among the merge candidates included inthe merge candidate list.

3-4) Inter Prediction in Skip Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing a merge candidate indicated by a skip index, among the mergecandidates included in a merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the skip index.

4) Current Picture Reference Mode

The current picture reference mode may denote a prediction mode thatuses a previously reconstructed region in a target picture to which atarget block belongs.

A motion vector for specifying the previously reconstructed region maybe used. Whether the target block has been encoded in the currentpicture reference mode may be determined using the reference pictureindex of the target block.

A flag or index indicating whether the target block is a block encodedin the current picture reference mode may be signaled by the encodingapparatus 100 to the decoding apparatus 200. Alternatively, whether thetarget block is a block encoded in the current picture reference modemay be inferred through the reference picture index of the target block.

When the target block is encoded in the current picture reference mode,the target picture may exist at a fixed location or an arbitrarylocation in a reference picture list for the target block.

For example, the fixed location may be either a location where a valueof the reference picture index is 0 or the last location.

When the target picture exists at an arbitrary location in the referencepicture list, an additional reference picture index indicating such anarbitrary location may be signaled by the encoding apparatus 100 to thedecoding apparatus 200.

In the above-described AMVP mode, merge mode, and skip mode, motioninformation to be used for the prediction of a target block may bespecified, among pieces of motion information in the list, using theindex of the list.

In order to improve encoding efficiency, the encoding apparatus 100 maysignal only the index of an element that incurs the minimum cost ininter prediction of the target block, among elements in the list. Theencoding apparatus 100 may encode the index, and may signal the encodedindex.

Therefore, the above-described lists (i.e. the prediction motion vectorcandidate list and the merge candidate list) must be able to be derivedby the encoding apparatus 100 and the decoding apparatus 200 using thesame scheme based on the same data. Here, the same data may include areconstructed picture and a reconstructed block. Further, in order tospecify an element using an index, the order of the elements in the listmust be fixed.

FIG. 10 illustrates spatial candidates according to an embodiment.

In FIG. 10, the locations of spatial candidates are illustrated.

The large block in the center of the drawing may denote a target block.Five small blocks may denote spatial candidates.

The coordinates of the target block may be (xP, yP), and the size of thetarget block may be represented by (nPSW, nPSH).

Spatial candidate A₀ may be a block adjacent to the below-left corner ofthe target block. A₀ may be a block that occupies pixels located atcoordinates (xP−1, yP+nPSH+1).

Spatial candidate A₁ may be a block adjacent to the left of the targetblock. A₁ may be a lowermost block, among blocks adjacent to the left ofthe target block. Alternatively, A₁ may be a block adjacent to the topof A₀. A₁ may be a block that occupies pixels located at coordinates(xP−1, yP+nPSH).

Spatial candidate B₀ may be a block adjacent to the above-right cornerof the target block. B₀ may be a block that occupies pixels located atcoordinates (xP+nPSW+1, yP−1).

Spatial candidate B₁ may be a block adjacent to the top of the targetblock. B₁ may be a rightmost block, among blocks adjacent to the top ofthe target block. Alternatively, B₁ may be a block adjacent to the leftof B₀. B₁ may be a block that occupies pixels located at coordinates(xP+nPSW, yP−1).

Spatial candidate B₂ may be a block adjacent to the above-left corner ofthe target block. B₂ may be a block that occupies pixels located atcoordinates (xP−1, yP−1).

Determination of Availability of Spatial Candidate and TemporalCandidate

In order to include the motion information of a spatial candidate or themotion information of a temporal candidate in a list, it must bedetermined whether the motion information of the spatial candidate orthe motion information of the temporal candidate is available.

Hereinafter, a candidate block may include a spatial candidate and atemporal candidate.

For example, the determination may be performed by sequentially applyingthe following steps 1) to 4).

Step 1) When a PU including a candidate block is out of the boundary ofa picture, the availability of the candidate block may be set to“false”. The expression “availability is set to false” may have the samemeaning as “set to be unavailable”.

Step 2) When a PU including a candidate block is out of the boundary ofa slice, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differentslices, the availability of the candidate block may be set to “false”.

Step 3) When a PU including a candidate block is out of the boundary ofa tile, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differenttiles, the availability of the candidate block may be set to “false”.

Step 4) When the prediction mode of a PU including a candidate block isan intra-prediction mode, the availability of the candidate block may beset to “false”. When a PU including a candidate block does not use interprediction, the availability of the candidate block may be set to“false”.

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment.

As shown in FIG. 11, when pieces of motion information of spatialcandidates are added to a merge list, the order of A₁, B₁, B₀, A₀, andB₂ may be used. That is, pieces of motion information of availablespatial candidates may be added to the merge list in the order of A₁,B₁, B₀, A₀, and B₂.

Method for Deriving Merge List in Merge Mode and Skip Mode

As described above, the maximum number of merge candidates in the mergelist may be set. The set maximum number is indicated by “N”. The setnumber may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200. The slice header of a slice may include N. Inother words, the maximum number of merge candidates in the merge listfor the target block of the slice may be set by the slice header. Forexample, the value of N may be basically 5.

Pieces of motion information (i.e., merge candidates) may be added tothe merge list in the order of the following steps 1) to 4).

Step 1) Among spatial candidates, available spatial candidates may beadded to the merge list. Pieces of motion information of the availablespatial candidates may be added to the merge list in the orderillustrated in FIG. 10. Here, when the motion information of anavailable spatial candidate overlaps other motion information alreadypresent in the merge list, the motion information may not be added tothe merge list. The operation of checking whether the correspondingmotion information overlaps other motion information present in the listmay be referred to in brief as an “overlap check”.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in the mergelist is less than N and a temporal candidate is available, the motioninformation of the temporal candidate may be added to the merge list.Here, when the motion information of the available temporal candidateoverlaps other motion information already present in the merge list, themotion information may not be added to the merge list.

Step 3) When the number of pieces of motion information in the mergelist is less than N and the type of a target slice is “B”, combinedmotion information generated by combined bidirectional prediction(bi-prediction) may be added to the merge list.

The target slice may be a slice including a target block.

The combined motion information may be a combination of L0 motioninformation and L1 motion information. L0 motion information may bemotion information that refers only to a reference picture list L0. L1motion information may be motion information that refers only to areference picture list L1.

In the merge list, one or more pieces of L0 motion information may bepresent. Further, in the merge list, one or more pieces of L1 motioninformation may be present.

The combined motion information may include one or more pieces ofcombined motion information. When the combined motion information isgenerated, L0 motion information and L1 motion information, which are tobe used for generation, among the one or more pieces of L0 motioninformation and the one or more pieces of L1 motion information, may bepredefined. One or more pieces of combined motion information may begenerated in a predefined order via combined bidirectional prediction,which uses a pair of different pieces of motion information in the mergelist. One of the pair of different pieces of motion information may beL0 motion information and the other of the pair may be L1 motioninformation.

For example, combined motion information that is added with the highestpriority may be a combination of L0 motion information having a mergeindex of 0 and L1 motion information having a merge index of 1. Whenmotion information having a merge index of 0 is not L0 motioninformation or when motion information having a merge index of 1 is notL1 motion information, the combined motion information may be neithergenerated nor added. Next, the combined motion information that is addedwith the next priority may be a combination of L0 motion information,having a merge index of 1, and L1 motion information, having a mergeindex of 0. Subsequent detailed combinations may conform to othercombinations of video encoding/decoding fields.

Here, when the combined motion information overlaps other motioninformation already present in the merge list, the combined motioninformation may not be added to the merge list.

Step 4) When the number of pieces of motion information in the mergelist is less than N, motion information of a zero vector may be added tothe merge list.

The zero-vector motion information may be motion information for whichthe motion vector is a zero vector.

The number of pieces of zero-vector motion information may be one ormore. The reference picture indices of one or more pieces of zero-vectormotion information may be different from each other. For example, thevalue of the reference picture index of first zero-vector motioninformation may be 0. The value of the reference picture index of secondzero-vector motion information may be 1.

The number of pieces of zero-vector motion information may be identicalto the number of reference pictures in the reference picture list.

The reference direction of zero-vector motion information may bebidirectional. Both of the motion vectors may be zero vectors. Thenumber of pieces of zero-vector motion information may be the smallerone of the number of reference pictures in the reference picture list L0and the number of reference pictures in the reference picture list L1.Alternatively, when the number of reference pictures in the referencepicture list L0 and the number of reference pictures in the referencepicture list L1 are different from each other, a reference directionthat is unidirectional may be used for a reference picture index thatmay be applied only to a single reference picture list.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add the zero-vector motion information to the merge listwhile changing the reference picture index.

When zero-vector motion information overlaps other motion informationalready present in the merge list, the zero-vector motion informationmay not be added to the merge list.

The order of the above-described steps 1) to 4) is merely exemplary, andmay be changed. Further, some of the above steps may be omitteddepending on predefined conditions.

Method for Deriving Prediction Motion Vector Candidate List in AMVP Mode

The maximum number of prediction motion vector candidates in aprediction motion vector candidate list may be predefined. Thepredefined maximum number is indicated by N. For example, the predefinedmaximum number may be 2.

Pieces of motion information (i.e. prediction motion vector candidates)may be added to the prediction motion vector candidate list in the orderof the following steps 1) to 3).

Step 1) Available spatial candidates, among spatial candidates, may beadded to the prediction motion vector candidate list. The spatialcandidates may include a first spatial candidate and a second spatialcandidate.

The first spatial candidate may be one of A₀, A₁, scaled A₀, and scaledA₁. The second spatial candidate may be one of B₀, B₁, B₂, scaled B₀,scaled B₁, and scaled B₂.

Pieces of motion information of available spatial candidates may beadded to the prediction motion vector candidate list in the order of thefirst spatial candidate and the second spatial candidate. In this case,when the motion information of an available spatial candidate overlapsother motion information already present in the prediction motion vectorcandidate list, the motion information may not be added to theprediction motion vector candidate list. In other words, when the valueof N is 2, if the motion information of a second spatial candidate isidentical to the motion information of a first spatial candidate, themotion information of the second spatial candidate may not be added tothe prediction motion vector candidate list.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in theprediction motion vector candidate list is less than N and a temporalcandidate is available, the motion information of the temporal candidatemay be added to the prediction motion vector candidate list. In thiscase, when the motion information of the available temporal candidateoverlaps other motion information already present in the predictionmotion vector candidate list, the motion information may not be added tothe prediction motion vector candidate list.

Step 3) When the number of pieces of motion information in theprediction motion vector candidate list is less than N, zero-vectormotion information may be added to the prediction motion vectorcandidate list.

The zero-vector motion information may include one or more pieces ofzero-vector motion information. The reference picture indices of the oneor more pieces of zero-vector motion information may be different fromeach other.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add pieces of zero-vector motion information to theprediction motion vector candidate list while changing the referencepicture index.

When zero-vector motion information overlaps other motion informationalready present in the prediction motion vector candidate list, thezero-vector motion information may not be added to the prediction motionvector candidate list.

The description of the zero-vector motion information, made above inconnection with the merge list, may also be applied to zero-vectormotion information. A repeated description thereof will be omitted.

The order of the above-described steps 1) to 3) is merely exemplary, andmay be changed. Further, some of the steps may be omitted depending onpredefined conditions.

FIG. 12 illustrates a transform and quantization process according to anexample.

As illustrated in FIG. 12, quantized levels may be generated byperforming a transform and/or quantization process on a residual signal.

A residual signal may be generated as the difference between an originalblock and a prediction block. Here, the prediction block may be a blockgenerated via intra prediction or inter prediction.

The residual signal may be transformed into a signal in a frequencydomain through a transform procedure that is a part of a quantizationprocedure.

A transform kernel used for a transform may include various DCT kernels,such as Discrete Cosine Transform (DCT) type 2 (DCT-II) and DiscreteSine Transform (DST) kernels.

These transform kernels may perform a separable transform or atwo-dimensional (2D) non-separable transform on the residual signal. Theseparable transform may be a transform indicating that a one-dimensional(1D) transform is performed on the residual signal in each of ahorizontal direction and a vertical direction.

The DCT type and the DST type, which are adaptively used for a 1Dtransform, may include DCT-V, DCT-VIII, DST-I, and DST-VII in additionto DCT-II, as shown in the following Table 3.

TABLE 3 Transform set Transform candidates 0 DST-VII, DCT-VIII 1DST-VII, DST-I 2 DST-VII, DCT-V

As shown in Table 3, when a DCT type or a DST type to be used for atransform is derived, transform sets may be used. Each transform set mayinclude multiple transform candidates. Each transform candidate may be aDCT type or a DST type.

The following Table 4 shows examples of a transform set that is appliedto a horizontal direction depending on the intra-prediction mode.

TABLE 4 Intra-prediction mode Transform set 0 2 1 1 2 0 3 1 4 0 5 1 6 07 1 8 0 9 1 10 0 11 1 12 0 13 1 14 2 15 2 16 2 17 2 18 2 19 2 20 2 21 222 2 23 1 24 0 25 1 26 0 27 1 28 0 29 1 30 0 31 1 32 0 33 1

In Table 4, the number of each transform set to be applied to thehorizontal direction of a residual signal is indicated depending on theintra-prediction mode of the target block.

The following Table 5 shows examples of a transform set that is appliedto the vertical direction of the residual signal depending on theintra-prediction mode.

TABLE 5 Intra-prediction mode Transform set 0 2 1 1 2 0 3 1 4 0 5 1 6 07 1 8 0 9 1 10 0 11 1 12 0 13 1 14 0 15 0 16 0 17 0 18 0 19 0 20 0 21 022 0 23 1 24 0 25 1 26 0 27 1 28 0 29 1 30 0 31 1 32 0 33 1

As exemplified in FIGS. 4 and 5, transform sets to be applied to thehorizontal direction and the vertical direction may be predefineddepending on the intra-prediction mode of the target block. The encodingapparatus 100 may perform a transform and an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block. Further,the decoding apparatus 200 may perform an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block.

In the transform and inverse transform, transform sets to be applied tothe residual signal may be determined, as exemplified in Tables 3, 4,and 5, and may not be signaled. Transform indication information may besignaled from the encoding apparatus 100 to the decoding apparatus 200.The transform indication information may be information indicating whichone of multiple transform candidates included in the transform set to beapplied to the residual signal is used.

As described above, methods using various transforms may be applied to aresidual signal generated via intra prediction or inter prediction.

The transform may include at least one of a first transform and asecondary transform. A transform coefficient may be generated byperforming the first transform on the residual signal, and a secondarytransform coefficient may be generated by performing the secondarytransform on the transform coefficient.

The first transform may be referred to as a “primary transform”.Further, the first transform may also be referred to as an “AdaptiveMultiple Transform (AMT) scheme”. AMT may mean that, as described above,different transforms are applied to respective 1D directions (i.e. avertical direction and a horizontal direction).

A secondary transform may be a transform for improving energyconcentration on a transform coefficient generated by the firsttransform. Similar to the first transform, the secondary transform maybe a separable transform or a non-separable transform. Such anon-separable transform may be a Non-Separable Secondary Transform(NSST).

The first transform may be performed using at least one of predefinedmultiple transform methods. For example, the predefined multipletransform methods may include a Discrete Cosine Transform (DCT), aDiscrete Sine Transform (DST), a Karhunen-Loeve Transform (KLT), etc.

The secondary transform may be performed on the transform coefficientgenerated by performing the first transform.

A first transform and a secondary transform may be applied to signalcomponents corresponding to one or more of a luminance (luma) componentand a chrominance (chroma) component. Whether to apply the firsttransform and/or the secondary transform may be determined depending onat least one of coding parameters for a target block and/or aneighboring block. For example, whether to apply the first transformand/or the secondary transform may be determined depending on the sizeand/or shape of the target block.

The transform method(s) to be applied to a first transform and/or asecondary transform may be determined depending on at least one ofcoding parameters for a target block and/or a neighboring block. Thedetermined transform method may also indicate that a first transformand/or a secondary transform are not used.

Alternatively, transform information indicating a transform method maybe signaled from the encoding apparatus 100 to the decoding apparatus200. For example, the transform information may include the index of atransform to be used for a first transform and/or a secondary transform.

The quantized levels may be generated by performing quantization on theresult, generated by performing the primary transform and/or thesecondary transform, or on the residual signal.

FIG. 13 illustrates diagonal scanning according to an example.

FIG. 14 illustrates horizontal scanning according to an example.

FIG. 15 illustrates vertical scanning according to an example.

Quantized transform coefficients may be scanned via at least one of(up-right) diagonal scanning, vertical scanning, and horizontal scanningdepending on at least one of an intra-prediction mode, a block size, anda block shape. The block may be a Transform Unit (TU).

Each scanning may be initiated at a specific start point, and may beterminated at a specific end point.

For example, quantized transform coefficients may be changed to 1Dvector forms by scanning the coefficients of a block using diagonalscanning of FIG. 13. Alternatively, horizontal scanning of FIG. 14 orvertical scanning of FIG. 15, instead of diagonal scanning, may be useddepending on the size and/or intra-prediction mode of a block.

Vertical scanning may be the operation of scanning 2D block-typecoefficients in a column direction. Horizontal scanning may be theoperation of scanning 2D block-type coefficients in a row direction.

In other words, which one of diagonal scanning, vertical scanning, andhorizontal scanning is to be used may be determined depending on thesize and/or inter-prediction mode of the block.

As illustrated in FIGS. 13, 14, and 15, the quantized transformcoefficients may be scanned along a diagonal direction, a horizontaldirection or a vertical direction.

The quantized transform coefficients may be represented by block shapes.Each block may include multiple sub-blocks. Each sub-block may bedefined depending on a minimum block size or a minimum block shape.

In scanning, a scanning sequence depending on the type or direction ofscanning may be primarily applied to sub-blocks. Further, a scanningsequence depending on the direction of scanning may be applied toquantized transform coefficients in each sub-block.

For example, as illustrated in FIGS. 13, 14, and 15, when the size of atarget block is 8×8, quantized transform coefficients may be generatedthrough a primary transform, a secondary transform, and quantization onthe residual signal of the target block. Therefore, one of three typesof scanning sequences may be applied to four 4×4 sub-blocks, andquantized transform coefficients may also be scanned for each 4×4sub-block depending on the scanning sequence.

The scanned quantized transform coefficients may be entropy-encoded, anda bitstream may include the entropy-encoded quantized transformcoefficients.

The decoding apparatus 200 may generate quantized transform coefficientsvia entropy decoding on the bitstream. The quantized transformcoefficients may be aligned in the form of a 2D block via inversescanning Here, as the method of inverse scanning, at least one ofup-right diagonal scanning, vertical scanning, and horizontal scanningmay be performed.

Dequantization may be performed on the quantized transform coefficients.A secondary inverse transform may be performed on the result generatedby performing dequantization depending on whether to perform thesecondary inverse transform. Further, a primary inverse transform may beperformed on the result generated by performing the secondary inversetransform depending on whether the primary inverse transform is to beperformed. A reconstructed residual signal may be generated byperforming the primary inverse transform on the result generated byperforming the secondary inverse transform.

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment.

An encoding apparatus 1600 may correspond to the above-describedencoding apparatus 100.

The encoding apparatus 1600 may include a processing unit 1610, memory1630, a user interface (UI) input device 1650, a UI output device 1660,and storage 1640, which communicate with each other through a bus 1690.The encoding apparatus 1600 may further include a communication unit1620 coupled to a network 1699.

The processing unit 1610 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1630 or the storage 1640. The processing unit 1610 may be atleast one hardware processor.

The processing unit 1610 may generate and process signals, data orinformation that are input to the encoding apparatus 1600, are outputfrom the encoding apparatus 1600, or are used in the encoding apparatus1600, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1610.

The processing unit 1610 may include an inter-prediction unit 110, anintra-prediction unit 120, a switch 115, a subtractor 125, a transformunit 130, a quantization unit 140, an entropy encoding unit 150, adequantization unit 160, an inverse transform unit 170, an adder 175, afilter unit 180, and a reference picture buffer 190.

At least some of the inter-prediction unit 110, the intra-predictionunit 120, the switch 115, the subtractor 125, the transform unit 130,the quantization unit 140, the entropy encoding unit 150, thedequantization unit 160, the inverse transform unit 170, the adder 175,the filter unit 180, and the reference picture buffer 190 may be programmodules, and may communicate with an external device or system. Theprogram modules may be included in the encoding apparatus 1600 in theform of an operating system, an application program module, or otherprogram modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the encoding apparatus 1200.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the encoding apparatus 1600.

The processing unit 1610 may execute instructions or code in theinter-prediction unit 110, the intra-prediction unit 120, the switch115, the subtractor 125, the transform unit 130, the quantization unit140, the entropy encoding unit 150, the dequantization unit 160, theinverse transform unit 170, the adder 175, the filter unit 180, and thereference picture buffer 190.

A storage unit may denote the memory 1630 and/or the storage 1640. Eachof the memory 1630 and the storage 1640 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1630 mayinclude at least one of Read-Only Memory (ROM) 1631 and Random AccessMemory (RAM) 1632.

The storage unit may store data or information used for the operation ofthe encoding apparatus 1600. In an embodiment, the data or informationof the encoding apparatus 1600 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The encoding apparatus 1600 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the encoding apparatus 1600. The memory 1630 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1610.

Functions related to communication of the data or information of theencoding apparatus 1600 may be performed through the communication unit1220.

For example, the communication unit 1620 may transmit a bitstream to adecoding apparatus 1600, which will be described later.

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment.

The decoding apparatus 1700 may correspond to the above-describeddecoding apparatus 200.

The decoding apparatus 1700 may include a processing unit 1710, memory1730, a user interface (UI) input device 1750, a UI output device 1760,and storage 1740, which communicate with each other through a bus 1790.The decoding apparatus 1700 may further include a communication unit1720 coupled to a network 1399.

The processing unit 1710 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1730 or the storage 1740. The processing unit 1710 may be atleast one hardware processor.

The processing unit 1710 may generate and process signals, data orinformation that are input to the decoding apparatus 1700, are outputfrom the decoding apparatus 1700, or are used in the decoding apparatus1700, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1710.

The processing unit 1710 may include an entropy decoding unit 210, adequantization unit 220, an inverse transform unit 230, anintra-prediction unit 240, an inter-prediction unit 250, a switch 245,an adder 255, a filter unit 260, and a reference picture buffer 270.

At least some of the entropy decoding unit 210, the dequantization unit220, the inverse transform unit 230, the intra-prediction unit 240, theinter-prediction unit 250, the adder 255, the switch 245, the filterunit 260, and the reference picture buffer 270 of the decoding apparatus200 may be program modules, and may communicate with an external deviceor system. The program modules may be included in the decoding apparatus1700 in the form of an operating system, an application program module,or other program modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the decoding apparatus 1700.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the decoding apparatus 1700.

The processing unit 1710 may execute instructions or code in the entropydecoding unit 210, the dequantization unit 220, the inverse transformunit 230, the intra-prediction unit 240, the inter-prediction unit 250,the switch 245, the adder 255, the filter unit 260, and the referencepicture buffer 270.

A storage unit may denote the memory 1730 and/or the storage 1740. Eachof the memory 1730 and the storage 1740 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1730 mayinclude at least one of ROM 1731 and RAM 1732.

The storage unit may store data or information used for the operation ofthe decoding apparatus 1700. In an embodiment, the data or informationof the decoding apparatus 1700 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The decoding apparatus 1700 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the decoding apparatus 1700. The memory 1730 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1710.

Functions related to communication of the data or information of thedecoding apparatus 1700 may be performed through the communication unit1720.

For example, the communication unit 1720 may receive a bitstream fromthe encoding apparatus 1700.

Quantization Method and Apparatus for Improving Perceptual Image Quality

FIG. 18 illustrates traditional distortion and perceptual distortionaccording to an example.

In FIG. 18, a relationship between the bit rate and distortion of avideo is depicted.

As illustrated in FIG. 18, traditional distortion is indicated by acurved line. In contrast, perceptual distortion is indicated by astepped line.

Such perceptual distortion may be distortion actually experienced by aviewer about a video which is decoded and displayed.

In relation to traditional distortion, a statistically lossy region anda statistically lossless region are depicted.

In relation to perceptual distortion, a perceptually lossy region and aperceptually lossless region are depicted.

In accordance with a perceptual distortion graph, a viewer may suddenlyperceive great distortion at a point at which the bit rate is ‘a’. Incontrast, the viewer may not perceive a change in the degree ofdistortion in a region between the point at which the bit rate is ‘a’and the point at which the bit rate is ‘b’. Therefore, it may beprofitable to perform encoding at a bit rate of ‘a’ in the regionbetween ‘a’ and ‘b’, if possible, from the standpoint of perceptualimage quality by finely adjusting the bit rate in the region between ‘a’and ‘b’.

Also, in a video having a low bit rate, fine adjustment for perceptualimage quality may be required based on the principle of this perceptualdistortion.

FIG. 19 is a graph illustrating a relationship between the quantizationparameter and the quantization step of a fixed-rate step quantizationmethod.

In the following embodiment, multiple quantization methods will bedescribed.

The multiple quantization methods may include 1) a fixed-rate stepquantization method and 2) a variable-rate step quantization method.

The fixed-rate step quantization method may be a quantization method inwhich an increment in a quantization step depending on an increase inthe value of a quantization parameter by 1 is fixed. For example, thefixed-rate stepped quantization method may be a quantization method inwhich, whenever the value of the quantization parameter is increased by1, a quantization step is increased by about 12%.

For example, as illustrated in FIG. 19, when the fixed-rate stepquantization method is used, the quantization step may be doubled whenthe value of the quantization parameter is increased by 6.

When the fixed-rate step quantization method is used, the increment inthe quantization step may be smaller in a region in which the value ofthe quantization parameter is relatively small.

Hereinafter, the terms “increment in a quantization step” and “rate ofincrease in a quantization step” may be used to have the same meaning,and may be used interchangeably with each other.

Generally, the region in which the value of the quantization parameteris relatively small may be a high-image quality region having a high bitrate. Therefore, when the fixed-rate step quantization method is used, achange in perceptual image quality depending on a change in the value ofthe quantization parameter may be smaller in the range of a high bitrate. Further, since the change in perceptual image quality is small, itis easy to precisely control image quality, but the change in imagequality may be insignificant.

Also, when the fixed-rate step quantization method is used, theincrement in the quantization step may be larger in a region in whichthe value of the quantization parameter is relatively large.

Generally, the region in which the value of the quantization parameteris relatively large may be a low-image quality region having a low bitrate. Therefore, when the fixed-rate step quantization method is used, achange in perceptual image quality depending on a change in the value ofthe quantization parameter may be larger in the range of a low bit rate.Further, since the change in perceptual image quality is larger, it maybe difficult to precisely control image quality.

The variable-rate step quantization method may be a quantization methodin which an increment in a quantization step depending on an increase inthe value of the quantization parameter by 1 is not fixed.Alternatively, the variable-rate step quantization method may be aquantization method in which the increment in the quantization stepdepending on the increase in the value of the quantization parameter by1 varies with the value of the quantization parameter.

Since the increment in the quantization step depending on the increasein the value of the quantization parameter by 1 varies with the value ofthe quantization parameter, the perceptual image quality may be easilycontrolled in all bit rate regions when the variable-rate stepquantization method is used. From this aspect, the variable-rate stepquantization method may be regarded as a suitable quantization methodfrom the standpoint of perceptual image quality.

For example, as the value of the quantization parameter is smaller, theincrement in the quantization step of the variable-rate stepquantization method depending on the increase in the value of thequantization parameter by 1 may be larger.

For example, in the region in which the value of the quantizationparameter is relatively small, the increment in the quantization step ofthe variable-rate step quantization method may be greater than theincrement in the quantization step of the fixed-rate step quantizationmethod.

For example, in a region in which the value of the quantizationparameter is relatively large, the increment in the quantization step ofthe variable-rate step quantization method may be less than theincrement in the quantization step of the fixed-rate step quantizationmethod.

FIG. 20 is a graph illustrating a relationship between a quantizationparameter and a Signal-to-Noise Ratio (SNR) in a fixed-rate stepquantization method according to an example.

FIG. 20 shows that the quantization parameter and the SNR in thefixed-rate step quantization method have a linear relationship. By thisrelationship, linear control may be possible, and the SNR may be easilycontrolled.

Hereinafter, a description of a quantization method may also be appliedto an inverse quantization (dequantization) method. Quantization anddequantization may be processes respectively performed by the encodingapparatus 100 and the decoding apparatus 200 in order to performencoding and decoding of a target block, and may be understood tocorrespond to each other. For example, multiple dequantization methodsmay include, for example, a fixed-rate step dequantization method and avariable-rate step dequantization method. A description of thevariable-rate step quantization method may be applied to thevariable-rate step dequantization method. A description of thefixed-rate step quantization method may also be applied to thefixed-rate step dequantization method.

FIG. 21 is a flowchart of a quantization method according to anembodiment.

At step 2110, the quantization unit 140 may select a quantizationmethod.

The quantization unit 140 may select a quantization method to be usedfor the transform coefficient of a target block from among multiplequantization methods.

The multiple quantization methods may include a fixed-rate stepquantization method and a variable-rate step quantization method.

A quantization method indicator may indicate the quantization methodselected from among the multiple quantization methods.

Below, the selected quantization method may be the variable-rate stepquantization method.

At step 2120, the quantization unit 140 may perform quantization thatuses the selected quantization method.

The quantization unit 140 may generate a quantized transform coefficientlevel by performing quantization that uses the selected quantizationmethod on the transform coefficient.

At step 2130, the entropy encoding unit 150 may store information aboutquantization.

The entropy encoding unit 150 may generate a bitstream includinginformation about quantization. Alternatively, the entropy encoding unit150 may include information about quantization in the bitstreamgenerated by the encoding apparatus 100.

The information about quantization may include 1) quantization methodindicator information, 2) quantization parameter information, and 3)quantized transform coefficient level information.

The quantization method indicator information may indicate aquantization method indicator. The quantization parameter informationmay indicate a quantization parameter. The quantized transformcoefficient level information may indicate a quantized transformcoefficient level.

The entropy encoding unit 150 may generate quantization method indicatorinformation by performing entropy encoding on the quantization methodindicator. The quantization method indicator information may begenerated between steps 2110 and 2120, and may be stored in a bitstream.

The entropy encoding unit 150 may generate quantization parameterinformation by performing entropy encoding on the quantizationparameter. The quantization parameter information may be generatedeither prior to step 2110 or between steps 2110 and 2120, and may bestored in the bitstream.

The entropy encoding unit 150 may generate quantized transformcoefficient level information by performing entropy encoding on thequantized transform coefficient level.

FIG. 22 is a flowchart of a dequantization method according to anembodiment.

The entropy decoding unit 210 may receive a bitstream.

The bitstream may include information about a target block. Theinformation about the target block may include information aboutquantization.

As described above with reference to FIG. 21, the quantizationinformation may include 1) quantization method indicator information, 2)quantization parameter information, and 3) quantized transformcoefficient level information.

The quantization method indicator may also be referred to as a“dequantization method indicator”. Further, the quantization parametermay also be referred to as a “dequantization parameter”.

At step 2210, the entropy decoding unit 210 may acquire a quantizationmethod indicator.

The quantization method indicator may indicate one dequantization methodthat is used for dequantization of a target block, among multipledequantization methods.

The entropy decoding unit 210 may acquire the quantization methodindicator by performing entropy decoding on the quantization methodindicator information.

At step 2220, the entropy decoding unit 210 may acquire a quantizationparameter.

The quantization parameter may be used for dequantization of the targetblock.

The entropy decoding unit 210 may acquire the quantization parameter byperforming entropy decoding on the quantization parameter information.Alternatively, the entropy decoding unit 210 may acquire thequantization parameter using a quantization parameter difference value,which will be described later.

At step 2230, the inverse quantization (dequantization) unit 220 mayselect a dequantization method.

The dequantization unit 220 may select a dequantization method,indicated by the quantization method indicator, from among multipledequantization methods.

At step 2240, the dequantization unit 220 may perform dequantizationthat uses the selected dequantization method.

The dequantization unit may perform dequantization using both theinformation about the target block and the dequantization method.

The dequantization unit 220 may generate a transform coefficient byperforming dequantization that uses the selected dequantization methodon the quantized transform coefficient level.

FIG. 23 illustrates a quantization step depending on a quantizationparameter according to an example.

In detail, FIG. 23 shows the modeling of a relationship between thequantization parameter and the quantization step in a variable-rate stepquantization method.

In FIG. 23, a relationship between the quantization parameter and thequantization step in a fixed-rate step quantization method is depictedas a graph.

In the graph, the x axis denotes the values of the quantizationparameter in the fixed-rate step quantization method. The y axis denotesthe values of the quantization step.

In FIG. 23, O symbols indicate the values of the quantization stepcorresponding to the values of the quantization parameter, with respectto integer values of the quantization parameter in the fixed-rate stepquantization method. In other words, the coordinates of a single Osymbol may be (the value of the quantization parameter in the fixed-ratestep quantization method, the value of the quantization stepcorresponding to the value of the quantization parameter in thefixed-rate step quantization method). Since the values of thequantization parameter in the fixed-rate step quantization method areintegers, and the x axis of the graph denotes the values of thequantization parameter in the fixed-rate step quantization method, the Osymbols are depicted at the integer positions of the x axis.

In FIG. 23, the X symbols denote the values of the quantization stepcorresponding to the integer values of the quantization parameter in thevariable-rate step quantization method. However, the X symbols (forcomparison with the fixed-rate step quantization method) are arranged tobe disposed on a curve defined by O symbols. In other words, the xcoordinate of each X symbol may not indicate the value of thequantization parameter in the variable-rate step quantization method.Two adjacent X symbols may correspond to integer values of thequantization parameter of the variable-rate step quantization method,which have a difference of 1 therebetween.

As illustrated in FIG. 23, in the fixed-rate step quantization method,as the value of the quantization parameter is increased by 1, thequantization step may be uniformly increased by about 12%. In otherwords, the difference between the quantization steps of the adjacent Osymbols may be about 12%.

As illustrated in FIG. 23, an interval between the X symbols may bewider than an interval between the O symbols in the range in which thevalues of the quantization parameter are relatively small. This may meanthat an increment in a quantization step defined by adjacent X symbolsis greater than an increment in a quantization step defined by adjacentO symbols. In other words, when the value of the quantization parameteris increased by 1 in the range in which the values of the quantizationparameter are relatively small, the increment in the quantization stepof the variable-rate step quantization method may be greater than theincrement in the quantization step of the fixed-rate step quantizationmethod.

Further, the interval between the X symbols may be narrower than theinterval between the O symbols in the range in which the values of thequantization parameter are relatively large. This may mean that theincrement in the quantization step defined by the adjacent X symbols isless than the increment in the quantization step defined by the adjacentO symbols. In other words, when the value of the quantization parameteris increased by 1 in the range in which the values of the quantizationparameter are relatively large, the increment in the quantization stepof the variable-rate step quantization method may be less than theincrement in the quantization step of the fixed-rate step quantizationmethod.

FIG. 24 illustrates a quantization parameter and a quantization steprate according to an example.

In the graph of FIG. 24, the x axis denotes the values of thequantization parameter. The y axis denotes the values of thequantization step rate. The quantization step rate may be a ratiobetween the quantization step of the variable-rate step quantizationmethod and the quantization step of the fixed-rate step quantizationmethod.

The quantization step rate may be derived by a gamma function, a caloricfunction, or a function having a form and/or properties similar thereto.

According to the graph of FIG. 24, the quantization step rate isincreased in the range in which the values of the quantization parameterare relatively small. In other words, in the range in which the valuesof the quantization parameter are relatively small, the quantizationstep of the variable-rate step quantization method may be increased morerapidly than the quantization step of the fixed-rate step quantizationmethod.

Further, in accordance with the graph of FIG. 24, in the range in whichthe values of the quantization parameter are relatively large, thequantization step rate is decreased. In other words, in the range inwhich the values of the quantization parameter are relatively large, thequantization step of the fixed-rate step quantization method may beincreased more rapidly than the quantization step of the variable-ratestep quantization method.

The quantization method implemented from the standpoint of perceptualimage quality may be practiced by defining and/or adjusting theincrement in the value of the quantization step depending on an increasein the value of the quantization parameter by 1, as described above.Such definition and/or adjustment may be implemented using the followingmethods 1) and 2).

1) The quantization parameter of a fixed-rate step quantization methodcorresponding to the quantization parameter of a variable-rate stepquantization method is determined, and a quantization step correspondingto the determined quantization parameter of the fixed-rate stepquantization method may be used for quantization.

2) A quantization step corresponding to a quantization parameter isdetermined based on a fixed-rate step quantization method, and aquantization step rate is applied to the determined quantization step,and thus the quantization step of a variable-rate step quantizationmethod may be determined. In other words, the determined quantizationstep may be the product of the quantization step of the fixed-rate stepquantization method and the quantization step rate.

Hereinafter, an example of a method of determining the quantizationparameter of a fixed-rate step quantization method corresponding to thequantization parameter of a variable-rate step quantization method andutilizing a quantization step corresponding to the determinedquantization parameter of the fixed-rate step quantization method willbe described.

Below, the quantization parameter of the variable-rate step quantizationmethod is indicated by QP_(perceptual), and the quantization parameterof the fixed-rate step quantization method is indicated by QP.

In order to use the variable-rate step quantization method which is aquantization method implemented from the standpoint of perceptual imagequality, QP_(perceptual) and QP may be mapped to each other usingvarious methods. Also, a formula or a relationship between the QP andthe quantization step used in the fixed-rate step quantization methodmay be defined and used. In accordance with such mapping andrelationship, a quantization step corresponding to QP_(perceptual) maybe defined and/or derived, and the quantization step corresponding toQP_(perceptual) may be used depending on the QP_(perceptual).

The following Equation (2) shows a gamma function indicating therelationship between QP_(perceptual) and QP according to an example.

$\begin{matrix}{{QP} = {\alpha \times \left( \frac{{QP}_{perceptual}}{\alpha} \right)^{\gamma}}} & (2)\end{matrix}$

QP_(perceptual) corresponding to a specific QP may be determineddepending on the relationship represented by the gamma function ofEquation (2).

When QP_(perceptual) is used for quantization and/or for dequantization,the value of QP corresponding to the value of QP_(perceptual) may bederived through the gamma function such as that shown in Equation (2).The gamma function may have gamma function parameters (α, γ). The valueof QP corresponding to the value of QP_(perceptual) may be determined bythe gamma function parameters (α, γ).

Fixed predefined values may be used as the gamma function parameters (α,γ) in the encoding apparatus 100 and the decoding apparatus 200. Inother words, the encoding apparatus 100 and the decoding apparatus 200may share the same gamma function parameters (α, γ) with each other.

Alternatively, the parameters (α, γ) of the gamma function may besignaled from the encoding apparatus 100 to the decoding apparatus 200through a bitstream.

For example, the above-described information about quantization mayinclude information about the gamma function parameters. The entropyencoding unit 150 may generate gamma function parameter information byperforming entropy encoding on the gamma function parameters of thegamma function. The entropy decoding unit 210 may acquire gamma functionparameters by performing entropy decoding on the gamma functionparameter information.

Of the gamma function parameters, parameter α may be a parameter whichenables the value of QP falling within an arbitrary range for the inputQP_(perceptual) to be issued. α may have an integer value or a valuescaled to an integer.

Of the gamma function parameters, parameter γ may be a parameter fordetermining QP corresponding to the input QP_(perceptual). γ may have aninteger value or a value scaled to an integer.

Generally, QP may have an integer value falling within the range from 0to 51. In contrast, the value of QP corresponding to the inputQP_(perceptual) may be a real number falling within the range from 0 to51. However, the value of the quantization step actually applied to thescaling of the transform coefficient may be an integer. The value of thequantization step may be a value made to approximate an integer formthrough scaling. In other words, even if the value of QP derived throughthe above-described method can be represented by a real number otherthan an integer, the integer value of the quantization step may bedetermined using various methods.

For example, the value of the quantization step may be scaled and madeto approximate an integer value using a related function between the QPand the quantization step. Alternatively, the value of the quantizationstep may be determined through table lookup.

FIG. 25 illustrates a table of quantization parameters and quantizationsteps of a variable-rate step quantization method according to anexample.

In FIG. 25, the values of a quantization parameter qP and a quantizationstep q_step used in quantization and dequantization, which use avariable-rate step quantization method, are depicted.

In FIG. 25, the values of a quantization step q_step corresponding tothe values of a specific quantization parameter qP are exemplified. Forexample, when the value of the quantization parameter is 0, the value ofthe quantization step may be 40.

The qP and q_step, illustrated in FIG. 25, may respectively indicateQP_(perceptual) and a quantization step corresponding toQP_(perceptual). In other words, the table shown in FIG. 25 may be usedfor lookup to determine the value of the above-described quantizationstep.

The quantization parameter may be used for each unit of encoding and/ordecoding of a video. For example, for a slice, the quantizationparameter may be determined. In the case of a luminance (luma)component, the quantization parameter determined for the slice may beSliceQpY. By means of the quantization parameter determined for theslice, the quantization parameter qP for a target block in the slice maybe determined.

The value of the quantization step corresponding to the quantizationparameter qP may be determined by methods including 1) table lookup forthe table, such as that shown in FIG. 25, 2) a relational expressionbetween the quantization parameter and the quantization step, and 3)various other methods.

For example, the value of the quantization step may be derived using arelated function between the increments (or increase rates) of thevalues of the quantization parameter and the quantization step.

For example, the value of the quantization step may be determined withreference to predefined values through table lookup, as illustrated inFIG. 25. The value of the quantization step may be determined throughtable lookup, as shown in the following Equation (3):

q_step=q_step_table[qP]  (3)

where q_step_table denotes a table.

For example, the value of the quantization step may be determined asrepresented by the following Equation (4), using a relational expressionbetween the quantization parameter and the quantization step.

q_step=q_function(qP,param1,param2, . . . )  (4)

Input parameters of q_function for determining the value of thequantization step may include 1) the value qP of the quantizationparameter in a variable-rate step quantization method and 2) one or moreparameters of a function for transforming the value qP of thequantization parameter in the variable-rate step quantization methodinto the value of the corresponding quantization step. For example, theone or more parameters may be the above-described α and γ.

As the input parameters are input, q_function may derive the value ofthe quantization step.

q_function may be implemented as a fixed-point operation in order todetermine a scaled value in a quantized integer form for the value qP ofthe input quantization parameter.

A dequantization method that uses the value of the quantization step maybe performed as represented by the following Equation (5):

dq_transCo effLevel=TransCoeff Level*m*q_step  (5)

TransCoeffLevel may denote a single quantized transform coefficient, mmay denote a scaling factor, and q_step may denote a quantization step.

Scaling may be performed on the single quantized transform coefficientTransCoeffLevel by the scaling factor m.

The dequantized transform coefficient dq_transCoeffLevel may be theproduct of the scaled quantized transform coefficient and thequantization step q_step. Alternatively, the dequantized transformcoefficient dq_transCoeffLevel may be the product of the quantizedtransform coefficient TransCoeffLevel, the scaling factor m for thequantized transform coefficient, and the quantization step.

The dequantized transform coefficient may be converted into or set to aninteger form through an operation in which data types of respectivevalues based on the implementation of the encoding apparatus 100 and thedecoding apparatus 200, dynamic ranges of respective values, etc. aretaken into consideration. Here, the values may include theabove-described TransCoeffLevel, m, and q_step. The operation mayinclude a clipping operation, a rounding operation, etc.

The above-described dequantization may be performed by thedequantization unit 160 of the encoding apparatus 100 and/or thedequantization unit 220 of the decoding apparatus 200. Further,quantization corresponding to the above-described dequantization may beperformed by the quantization unit 140 of the encoding apparatus 100.

FIG. 26 illustrates the syntax of a video parameter set according to anexample.

FIG. 27 illustrates the syntax of a sequence parameter set according toan example.

FIG. 28 illustrates the syntax of a picture parameter set according toan example.

FIG. 29 illustrates the syntax of a slice segment header according to anexample.

When dequantization is performed, the quantization method to be appliedto a specific unit may be indicated through upper-level syntax. Here,the indicated quantization method may be one quantization method, amongmultiple quantization methods. The upper-level syntax may include aquantization method indicator, and the quantization method indicatorincluded in the upper-level syntax may indicate the quantization methodto be applied to a specific unit, which is the target of the upper-levelsyntax.

Alternatively, the upper-level syntax may indicate whether avariable-rate step quantization method is applied to the specific unit.For example, a quantization method indicator included in the upper-levelsyntax may indicate whether a variable-rate step quantization method isapplied to a specific unit, which is the target of the upper-levelsyntax.

Alternatively, the upper-level syntax may indicate a quantization methodto be applied to the specific unit, among the multiple quantizationmethods.

For example, the specific unit may be a video, a sequence, a picture, ora slice.

For example, a video parameter set may indicate a quantization method tobe applied to the target of the video parameter set. The video parameterset may include the above-described quantization method indicatorinformation. The quantization method indicator information may indicatea quantization method to be applied to the target of the video parameterset, among multiple quantization methods. Alternatively, thequantization method indicator information may indicate whether avariable-rate step quantization method is applied to the target of thevideo parameter set.

As the quantization method indicator information for the video parameterset, vps_perceptual_qp_enabled_flag is exemplified in FIG. 26. Thequantization method indicator information may be a 1-bit flag.

For example, the sequence parameter set may indicate a quantizationmethod to be applied to the target of the sequence parameter set. Thesequence parameter set may include the above-described quantizationmethod indicator information. The quantization method indicatorinformation may indicate the quantization method to be applied to thetarget of the sequence parameter set, among multiple quantizationmethods. Alternatively, the quantization method indicator informationmay indicate whether a variable-rate step quantization method is appliedto the target of the sequence parameter set.

As the quantization method indicator information for the sequenceparameter set, sps_perceptual_qp_enabled_flag is exemplified in FIG. 27.The quantization method indicator information may be a 1-bit flag.

For example, the picture parameter set may indicate the quantizationmethod to be applied to the target of the picture parameter set. Thepicture parameter set may include the above-described quantizationmethod indicator information. The quantization method indicatorinformation may indicate the quantization method to be applied to thetarget of the picture parameter set, among multiple quantizationmethods. Alternatively, the quantization method indicator informationmay indicate whether a variable-rate step quantization method is appliedto the target of the picture parameter set.

As the quantization method indicator information for the above-describedpicture parameter set, pps_perceptual_qp_enabled_flag is exemplified inFIG. 28. The quantization method indicator information may be a 1-bitflag.

For example, the slice segment header may indicate the quantizationmethod to be applied to the target of the slice segment header. Theslice segment header may include the above-described quantization methodindicator information. The quantization method indicator information mayindicate the quantization method to be applied to the target of theslice segment header, among multiple quantization methods.Alternatively, the quantization method indicator information mayindicate whether a variable-rate step quantization method is applied tothe target of the slice segment header.

As the quantization method indicator information for the above-describedslice segment header, slice_segment_header_perceptual_qp_enabled_flag isexemplified in FIG. 29. The quantization method indicator informationmay be a 1-bit flag.

When dequantization is performed, the quantization parameter to beapplied to a specific unit may be indicated by upper-level syntax. Forexample, the upper-level syntax may be a video parameter set, a sequenceparameter set, a picture parameter set or a slice segment header.

For example, the video parameter set may include a quantizationparameter to be applied to the target of the video parameter set. Thesequence parameter set may include a quantization parameter to beapplied to the target of the sequence parameter set. The pictureparameter set may include a quantization parameter to be applied to thetarget of the picture parameter set. The slice segment header mayinclude a quantization parameter to be applied to the target of theslice segment header.

When a value of the quantization method indicator equals to a predefinedvalue (for example, “1”), the upper-level syntax may include additionalsyntax elements for the variable-rate step dequantization. Theadditional syntax elements may be used to configure values used in thevariable-rate step quantization method previously mentioned. Forexample, the additional syntax elements includes the gamma functionparameters (α, γ).

In order to indicate the quantization parameter in the variable-ratestep quantization method, a specific number of bits may be required. Toreduce the number of bits required, substitute information indicatingthe value of the quantization parameter may be used instead of directlysignaling the value of the quantization parameter. Such substituteinformation may be used for a specific unit, may be contained inupper-level syntax to be applied to the specific unit, and may then besignaled for each specific unit. Alternatively, information aboutquantization, described above with reference to FIGS. 21 and 22, mayinclude such substitute information.

For example, the substitute information for indicating the value of thequantization parameter may be a quantization parameter difference value.

For example, the quantization parameter difference value may be thedifference between a median value and the value of the quantizationparameter. Here, the median value may be an intermediate value fallingwithin the range of the quantization parameter of the variable-rate stepquantization method.

The quantization unit 140 may generate information about thequantization parameter difference value by performing entropy encodingthat uses signed exponential Golomb on the quantization parameterdifference value.

Alternatively, the quantization parameter difference value may be thedifference between the value of the quantization parameter in an upperunit and the value of the quantization parameter in a lower unit. Forexample, the upper unit may be a picture, and the lower unit may be aslice. The quantization parameter value of each unit may be signaledthrough the above-described signed exponential Golomb method. Thequantization parameter, acquired using the quantization parameterdifference value, may be used for quantization of a lower unit.

The quantization parameter difference value may be included in theheader of the lower unit.

For the target picture, information about a picture quantizationparameter difference value may be signaled. For example, the pictureparameter set may include the picture quantization parameter differencevalue. The picture quantization parameter difference value may beinformation used to derive a quantization parameter that is applied to apicture.

Further, for each of one or more slices in a target picture, informationabout a slice quantization parameter difference value may be signaled.The slice header of each slice may include the slice quantizationparameter difference value.

The slice quantization parameter difference value may indicate thedifference between the value of a quantization parameter that is appliedto the picture and the value of a quantization parameter that is appliedto the slice.

In the above-described embodiments, although the methods have beendescribed based on flowcharts as a series of steps or units, the presentdisclosure is not limited to the sequence of the steps and some stepsmay be performed in a sequence different from that of the describedsteps or simultaneously with other steps. Further, those skilled in theart will understand that the steps shown in the flowchart are notexclusive and may further include other steps, or that one or more stepsin the flowchart may be deleted without departing from the scope of thedisclosure.

The above-described embodiments according to the present disclosure maybe implemented as a program that can be executed by various computermeans and may be recorded on a computer-readable storage medium. Thecomputer-readable storage medium may include program instructions, datafiles, and data structures, either solely or in combination. Programinstructions recorded on the storage medium may have been speciallydesigned and configured for the present disclosure, or may be known toor available to those who have ordinary knowledge in the field ofcomputer software.

A computer-readable storage medium may include information used in theembodiments of the present disclosure. For example, thecomputer-readable storage medium may include a bitstream, and thebitstream may contain the information described above in the embodimentsof the present disclosure.

The computer-readable storage medium may include a non-transitorycomputer-readable medium.

Examples of the computer-readable storage medium include all types ofhardware devices specially configured to record and execute programinstructions, such as magnetic media, such as a hard disk, a floppydisk, and magnetic tape, optical media, such as compact disk (CD)-ROMand a digital versatile disk (DVD), magneto-optical media, such as afloptical disk, ROM, RAM, and flash memory. Examples of the programinstructions include machine code, such as code created by a compiler,and high-level language code executable by a computer using aninterpreter. The hardware devices may be configured to operate as one ormore software modules in order to perform the operation of the presentdisclosure, and vice versa.

As described above, although the present disclosure has been describedbased on specific details such as detailed components and a limitednumber of embodiments and drawings, those are merely provided for easyunderstanding of the entire disclosure, the present disclosure is notlimited to those embodiments, and those skilled in the art will practicevarious changes and modifications from the above description.

Accordingly, it should be noted that the spirit of the presentembodiments is not limited to the above-described embodiments, and theaccompanying claims and equivalents and modifications thereof fallwithin the scope of the present disclosure.

What is claimed is:
 1. An encoding method, comprising: performingquantization that uses a quantization method, wherein the quantizationmethod is a variable-rate step quantization method, and wherein thevariable-rate step quantization method is a quantization method in whichan increment in a quantization step depending on an increase in a valueof a quantization parameter by 1 is not fixed.
 2. The encoding method ofclaim 1, wherein, as the value of the quantization parameter is smaller,the increment in the quantization step of the variable-rate stepquantization method depending on the increase in the value of thequantization parameter by 1 is larger.
 3. The encoding method of claim1, further comprising selecting the quantization method from amongmultiple quantization methods, wherein the multiple quantization methodscomprise a fixed-rate step quantization method and the variable-ratestep quantization method, and wherein the fixed-rate step quantizationmethod is a quantization method in which the increment in thequantization step depending on the increase in the value of thequantization parameter by 1 is fixed.
 4. The encoding method of claim 3,wherein the increment in the quantization step of the variable-rate stepquantization method is less than the increment in the quantization stepof the fixed-rate step quantization method in a region in which thevalue of the quantization parameter is relatively large.
 5. A decodingmethod, comprising: performing dequantization that uses a dequantizationmethod, wherein the dequantization method is a variable-rate stepdequantization method, and wherein the variable-rate step dequantizationmethod is a dequantization method in which an increment in aquantization step depending on an increase in a value of a quantizationparameter by 1 is not fixed.
 6. The decoding method of claim 5, wherein,as the value of the quantization parameter is smaller, the increment inthe quantization step of the variable-rate step dequantization methoddepending on the increase in the value of the quantization parameter by1 is larger.
 7. The decoding method of claim 5, further comprisingselecting the dequantization method from among multiple dequantizationmethods, wherein the multiple dequantization methods comprise afixed-rate step dequantization method and the variable-rate stepdequantization method, and wherein the fixed-rate step dequantizationmethod is a dequantization method in which the increment in thequantization step depending on the increase in the value of thequantization parameter by 1 is fixed.
 8. The decoding method of claim 7,wherein the increment in the quantization step of the variable-rate stepdequantization method is less than the increment in the quantizationstep of the fixed-rate step dequantization method in a region in whichthe value of the quantization parameter is relatively large.
 9. Thedecoding method of claim 7, wherein: a quantization parameter of thefixed-rate step dequantization method corresponding to the quantizationparameter of the variable-rate step dequantization method is determined,and a quantization step corresponding to the quantization parameter ofthe fixed-rate step dequantization method is used for thedequantization.
 10. The decoding method of claim 7, wherein: aquantization step of the fixed-rate step dequantization methodcorresponding to the quantization parameter is determined based on thefixed-rate step dequantization method, and a quantization step of thevariable-rate step dequantization method is determined by applying aquantization step rate to the quantization step of the fixed-rate stepdequantization method, and the quantization step rate is a ratio betweenthe quantization step of the variable-rate step dequantization methodand the quantization step of the fixed-rate step dequantization method.11. The decoding method of claim 5, further comprising acquiring aquantization method indicator, wherein the quantization method indicatorindicates one dequantization method to be used for dequantization of atarget block among the multiple dequantization methods.
 12. The decodingmethod of claim 11, wherein the quantization method indicator indicatesa dequantization method to be applied to a specific unit.
 13. Thedecoding method of claim 12, wherein the specific unit is a video, asequence, a picture or a slice.
 14. The decoding method of claim 12,wherein the quantization method indicator indicates whether thevariable-rate step dequantization method is applied to the specificunit.
 15. The decoding method of claim 5, further comprising acquiringthe quantization parameter using a quantization parameter differencevalue, wherein the quantization parameter is used for thedequantization, wherein the quantization parameter difference value is adifference between a median value and the value of the quantizationparameter, and wherein the median value is an intermediate value fallingwithin a range of the quantization parameter of the variable-rate stepdequantization method.
 16. The decoding method of claim 5, wherein thequantization parameter is applied to a specific unit.
 17. The decodingmethod of claim 5, further comprising acquiring the quantizationparameter using a quantization parameter difference value, wherein thequantization parameter difference value is a difference between a valueof a quantization parameter in an upper unit and a value of aquantization parameter in a lower unit, and wherein the acquiredquantization parameter is used for dequantization of the lower unit. 18.The decoding method of claim 17, wherein the quantization parameterdifference value is included in a header of the lower unit.
 19. Thedecoding method of claim 17, wherein the upper unit is a picture and thelower unit is a slice.
 20. A computer-readable storage medium storing abitstream for video decoding, the bitstream comprising: informationabout a target block, wherein dequantization that uses the informationabout the target block and a dequantization method is performed, whereinthe dequantization method is a variable-rate step dequantization method,and wherein the variable-rate step dequantization method is adequantization method in which an increment in a quantization stepdepending on an increase in a value of a quantization parameter by 1 isnot fixed.